The supposed £300 Fluval tank BigPig quotes is £370, with a poor filter included IMO. Fluval offer a seriously large discount to retailers (arround the 30% mark) if they buy more than 20 at a time (I know, I work in aquatics retail). I believe this is a big factor in why they are still about, as this discount makes them cheap-enough to get people to buy them, with a profit making them worth the while for the retailer, as they have lots of issues as they get towards the end of the warrenty period, as well as just outside...
You know the filter is poor if a manufacturer can't sell their kit without a discount. Eheim, Tetratec, Aquaone don't have to do that, and even the poor-quality Cascade filters don't do that bigger discount to get them away
The filter is the place where you should spend your cash, as it is the most important piece of kit in the tank. Fluval can be described as budget quality at best IMO. In the UK, Eheim are widely considered the best (avoid the Ecco range and the termo versions of any exturnal brand) and Tetratec make a good alternative on a tight budget. Tetratec are my favoured band ATM, but I won't give you my view of Eheim (it tends to caurse arguments)
Round my way, a new 50g tank can be bought new for about £400-£450ish with decent kit. Yes, a 50g is possible second hand for £300 with good kit. It would appear as though some have miss-read my first post
If building second-hand, pacience may be required, as good kit don't come round all that often. Bartering skills will also help save your budget.
Buy a filter rated for a tank far larger than the tank it is being bought for, as every manufacturer is "optimistic" about their filter's performance. Heaters must be the right size. Undersize and it won't get the tank to temperature, oversize and the temperature will fluctuate as the heater will raise the temperature too much before the thermostat swithes off the element. 50Watts over won't do a great deal, but enough heating for a 100g in a 50 will lead to a "bouncy" temperature
lighting depends on what plants (if any) you want to keep. A tank and stand are both farily strait foreward. On top of blue seals, look for any algea under the silicone or any signs of the silicone pealing/seal damage/tearing. Also, ask the age of any equipment when considering the purcahse. Know the expected life-span of the equipment you are considering, and make sure it still has life left in it
A damaged tank isn't always a scrap case. This is where experience comes in. Some things are fixable and others aren't. The ease of fixing can be used to barter, assuming you know how to fix the issue. Equipment is where you are most likely to be conned, as things can be covered up easily. Be weary of anything that isn't clean as this can hide stress fractures. Trust also plays a part. I diden't check over some filter's I bought of Miss Wiggle and her other half some time ago, as I trusted their word. If it were any other customer that wasn't a regular in the shop I work at, that I spoke to regularly, I'd have stripped the filter's first and checked everything under a good light, before re-assembling and testing with water. A genuine seller that knows the equipment is fine won't mind you doing this, but if they know something is bust...
HTH
Rabbut