It won't just be down to the filter that, goldfish are about the dirtiest fish you can keep!
I'd suggest a Rio 125 then as per gitbags post - the internal Juwel filter that comes with it will be more than adequate for your needs. There isn't a great deal to worry about with buying secondhand tanks, here's a few points for you:
- Check glass for scratches. Be aware that if the tank is full of water it will hide some scratches and marks.
- Check glass for any signs of cracks
- Check silicon for any signs of degradation (unless damaged it will last for years and years and years)
- Check the pump runs ok - just dunk the powerhead in some water and check it runs smooth - any rattling or similar (apart from clearing airlocks) shows a worn or damaged impeller or impeller shaft, and will cost you £10-20 to sort (not much, but significant on your budget!).
- Check the cabinet for any water marks/blistering - a lot of cabinets are just bog standard MDF, and once their protective outer surface is damaged, they're history.