best spec a race chassis as you say of you might want to go racing. ditch the spare wheel bracket anyway. Unnecessary weight, I just carry some tyre foam.
Yes a 1.6 can be upgraded to a 1.8 easily enough.
Also lots of good K's about as people in caterhams switch to duratec and elises switching to honda. Seen some bargains. My 5000mile ex-superlight K was a stunning bargain, a chap upgrading his superlight to duratec power! If you end up with a non-caterham K series don't forget to budget for all the other ancils you'll need, wet sump, bellhousing, flywheel, clutch, mods to block, starter, alternator, oil filter housing, water rail, crb, loom, hoses, cables etc.....
Much better to look for an ex-caterham K series with all these bits present and correct.