I was going to say £12.5k!!! That'll just about buy you an optimax and cables.
For the size you want I think twins is unlikely, by the time your setup.
I'd look for a good main engine like a 200hp Opti (no offence against evinrudes, they are bomb proof if not a little agricultural) as it will give you the performance your probably looking for.
I'd then invest in a 8hp auxilary engine for safety.
All this for £25k, your probably looking at something like an XS700 which will probably do all that you require. Although your budget maybe a little tight by the time you've sorted out console/seats/ and extras.
I've got (just about) the XS700 and can't recommend them enough although I am actually selling mine at the moment to buy the new XS950. I went for the delta formation as it gave me about 6ft of free space at the back of the RIB which is ideal for 2 or 3 divers to kit up.
Give Laurence at Barnet Marine a ring (or email....it's on the website www.xs-ribs.co.uk
) and see what he can do and say Ian from Guernsey sent you.