yes, an ST200 rear bumper is a direct replacement and fits straight on.
yes, twin exit system fits onto an ST24, bolts straight on to the cat.

I would recommend contacting Blueflame exhausts as they do off the shelf single and twin exit systems, both under £300. They sell direct and through ebay

http://www.blueflameperformance.com/
Tel: 01695 733100

http://stores.ebay.co.uk/BLUE-FLAME-PERFORMANCE

Whether you fit an ST24 or ST200 bumper, be aware, its is expensive
Many people modify thier existing bumper and have a 2nd hole put in, Blueflame will do this for you.

hope this helps, also, join www.stownersclub.com and have a nose around