http://www.cardomain.com/ride/2400656/2
thats a page i made ages ago on whats needed, have a look, it doesnt say everything but its alot of the stuff anyway.
the conversion is not cheap and if u want a 26 buy the 260rs because the difference wont pay for the conversion and u end up with a rs four with the gtr engine, then u need the gtr brakes, diff, suspension, ecu, intercooler, etc that all comes stock on the 260rs.
labour wouldnt be cheap either if u dont plan on doing it yourself. dont forget the 26's are all very very long in the tooth now and i wouldnt buy one without it being rebuilt or having it done once u get it as they dont last long and most are over the 150 000km mark regardless of the stupid odometer readings of 60 000km on every second import.
spend the extra on the 25 and keep the rs4 or sell and buy the 260rs and do some light mods. i still wouldnt trust the 26 to last very long, and pushing 350-400awhp through a stock used one will shorten its life span alot. rebuild it to stock and it will last on that power no worries but the old engines have worn rings and bearings from the rpm and wear and tear during there life.
thats just my personal opinion, hope it helps