Cracking question!

i will never own a 4x4 cosworth, if i do i break them, i built an escos, it was rwd too.

Answer to yer question, depends how far you wanna go?

front suspension/sump/rack/cradle/UJ`s are all different, you can swop the sump and use some nackered outer CV`s on the 4x4 setup hubs which is fine and creates no more drag than 2wd.

gearbox mount, you will have to made a midified one but no big deal.
speedo cable different aswell i think, not 100% on that one.

rear end, keep 4x4 brakes or full axle if you want but 7" little less reliable than 7.5" but not a lot! upto stg3 should be fine aslong as you dont smash the granny out of it in carparks and away from the lights on the limmiter and sidestepping the clutch!

prop will need to be T5 tho, unless you use MT75 RWD (also fine! i use scorpio hyd clutch ones for this on customers cars, again dont fcukin abuse them lol)

so depends if you want it 100% 2wd cos T5/7.5inch,
look on ebay for current buying and selling prices.