A tip? Buy mine!
*dodges bullets*
oh you meant a tip as in informed knowledge, not a dumping ground for household waste
ballpark cost? £200 to £800 depending on condition, miles, spec, etc. just my opinion/rough estimate though

try to haggle 'em down!
£200 or less would be a Fox or CL 1-litre, 4 speed, early 90s, with well over 100,000 on the clock and maybe scruffy bodywork/faded paint, one or two minor things needing to be done... but i'd still look for 6 months MOT/Tax with that (he said, talking his own sale into a corner)
£800 would be for a nice, mid-90s GT (1.3 high output, 5 speed), say about 50,000 miles, and good (though of course not perfect) bodywork and good mechanicals, 12 months tax/MOT ... (CL + GT sellers feel free to argue with me on that!)
If you want to get really sporty, you could go for a G40 or engine converted car, but it'd be highly inappropriate to your experience, you'd be completely helpless to find an insurer to cover you, and the board would probably be in uproar

(though, granted, i'm testosterone fuelled, i'm still going to give the example of damn near killing myself several times first in mums Punto 75 (basically more-or-less GT spec) and then in my own polo 1 litre.... the lower speeds offered by these cars being the main thing that saved my neck in most cases)
ebay is a good place to find bargain mk3s, if you find one in your area you can go give a damn good inspecting before bidding. other than that, autotrader, local classifieds, or keep a lookout in case a (trustworthy and established) UK Polos / Porka / oldskooldubs / etc member is selling off a well-loved example.
or just go cruise the streets - i actually spotted mine when out for a bike ride, just parked up with a sign in the window.
"quick" checklist:
no soot coming out of exhaust
no rust on petrol tank or (importantly) the filler neck leading into it. this will involve lying on the ground, i'm afraid.
no large patches of rust
anywhere, particularly outside of the replacable panels (remember to check inside - eg around spare tyre - as well as out). Some around the wheelarches is ok. also, cant speak for anyone else, but the metal separator for the quarterlight windows may be pure rust... doesn't seem to affect it adversely
no big oil leaks or soaking (or, a "suspiciously" clean engine)
not too low to the ground particularly at the back, unless it's been dropped. acceptable on a squareback hatch for the
very top of the tyre to be obscured inside the arch, but no further. as the boot is so big, it is possible to overload them and damage it.
starts very easily even from cold! (if its warm when you arrive --- alarm bells)... polo is the original "starts first time every time" car, when it's well.
idles calmly/smoothly after no more than the first minute.
steady temperature once warm of around 80-90 celcius. should take quite a while for the fan to come on.
if you check the heating system, dont expect hot air to come out of the face vents

as its designed to only blow warm from the screen/feet/side.
check door/boot seals for major leaks, and particularly the sunroof if it has one. not to mention the aerial if it's been relocated from the wing to the roof.
take a peek and listen to the exhaust to make sure it's not going to hit the ground and hasnt got a hole... including at the engine end as the seal can break down (rev it and listen for clacking noises)
and all the other usual car gubbins - goes into all gears easily (discounting a little bit of reverse crunch) and pulls along acceptibly in each, steers and brakes well, suspension isn't shot (bounce bodily up and down on each corner and see that it 'rebounds' once - no more no less), be wary of any odd engine or gearbox sounds and so on. look over interior (inc wheel, gearlever, pedals) and see if it matches your expectation of how worn it should be for the miles and age. should look fairly good if you find a 30,000 mile example...... though it'll be kind of lacking by 80,000! if it looks better than it should, that's no worry - but worse, that can be!
check the tyres are well inflated and have decent tread. do an emergency brake test and check the handbrake on a hill if you can (and... look at the lines in case they're rusted!). find a car park to do some full-lock turns in to listen for worn-out CV joints.
all that kind of stuff

best really to find a mate / relative / family friend who knows their onions to go along, but make sure you get versed in the relevant stuff yourself for anything they might miss. for example, when I bought mine, most things we ok --- but both me and three other people looking over it with me missed that the entire rear brake system was shredded, though all the signs were there if we'd looked just a little more slowly and carefully --- don't let the seller bully or rush you! (if i wasnt desperate for a car, i might have paid more attention to the handbrake handle being completely floppy)
Some things that are, however, polo typical: Stodgy and stiff controls, compared to modern cars. You might find the accelerator a bit resistant, the clutch heavy, the steering needs some work and the brakes a little firm (though, they're not actually as bad as you first imagine). Plus you will feel VERY low to the ground. Don't fret it. After about 10 miles of driving you settle right in and may come to appreciate the extra feedback and fine control that they afford.
PS if you are actually the back seat passenger on the test drives, don't be deceived by the squashy comfort of the rear bench - the front seats are a "little" harder than that, but, more supportive on corners (well.. unless it's a fox). Don't get trapped in it - remember that the door handle is in the "wrong" place (centre of the door car, not up closer to the front/top).
Oh, and if it's a 1-litre - the low revs are fine for trucking about town in, and it will feel, if not swift, then at least reasonably responsive (as unlike some small cars, it's got decent low-end torque). Just remember that to get any actual performance from that engine needs a lot of RPMs, so if it feels sluggish when you put your foot down... drop a gear and try to ignore the (eventual) howling noise!
Finally --- if you buy it --- be wary of the petrol guage. There's no warning light, and the accuracy can be variable to say the least. Fill it up and see how close to the "full" line the needle is, and try to keep at least that amount of distance from the "empty" red zone. I've run it dry with supposedly a gallon of reserve displayed before.
They're very nice cars, but, along the lines of the Beetle.... very basic. Don't expect much luxury/advanced comfort from it unless you do it up - but do expect a reliable, efficient and pleasurable drive!