Realistically - you cannot trade futures with $500. You'll have to use ridiculously tight stops and suffer from death of a thousand cuts.
In your case though, it won't actually take 1000 cuts.... Trade the ES - you'll be lucky to suffer 'death of 20 cuts"
Impossible on most. The absolute minimum in an account is around $500 because that;s the margin required for most contracts (maybe a bit less on bunds).
You will be automatically liquidated if your account gets below the margin.
Also, £3 for a trade is not too bad but it depends on what you trade.
A SB charging 3 pip spread at £1 a point will effectively be charging you £3.
Still, it's quite possible top open up an account with that and start but if you're new, I'd suggest trialling out a few things first. There are plenty of SBers offering £250-£500 loss covers for opening new accounts with them.
If I could do this all again, I'd have a much larger account than I first started with but you gotta start somewhere.
Also, go with a broker that offers less than £1 trade sizes - that will increase your effective account size / risk.