Volkswagen group cars (which incorporate Seat), are very reliable. Like any cars, with age comes wear but provided it's had a good, full history you should be safe. Don't let the apparent amount of faults on a forum fool you, just compare that to how many of us are driving cars without these issues and you'll soon see that it's a minor group who experience issues with their cars.
As for model, well, that's where it's tricky. Start by deciding which fuel you want/need and then go from there. That budget should buy you a well looked after late model mk5!
Manual or DSG is a long, never ending debate. Go drive both and see which you prefer.