Stevil

Ok,
I know this has been asked in one form or another several times. But, I have not found a thread w/ my specific wants. So here is my lengthy question:
I have stock twins w/ upgrades: CWR IC, HKS TWINAMP, 1200cc secondaries, large radiator, dual oil coolers, upgraded fuel pump, downpipe, hi-flow cat, Apexi N1 duals, water injection, 4.33 rear gearing, non-ported motor, petit intake and PFC(this is all the relavent stuff for my question).
I at first wanted to make at least 400(flywheel HP) w/ my car. Not sure where I am at now(and this is stilla goal)but, 400RWHP sounds good now(then 450 later and then 500), no 400rwhp(want to maintain a good driveability).
I want to keep my airpump, so have come to conclusion an HKS single turbo kit would be my match(any other options). They have the E and the R kits avail(I am not interested in the monster one ). I figure the diff between the two(besides HP) is the spool up time. Is it significant? Or actually, what can my present fuel system support? I do not want to upgrade fuel again or really anything else. I probably would only buy a kit if I could find a really good price(like used). I figure, i drive around w/ my higher RPMS due to my rear gearing, so spool up should not be a big factor , right?(plus all the talk of not noticing lag w/ a single from all you single guys).
Or would I feel a good increase in power(I know not 400rwhp, but) by just buying a set of upgraded twins(which I could get for less than $1800 from a private party) and leave it at that. I wouldnt have to have my vacuum line job, Petit intake(came w/ car), polished intake pipes. and IC inlet pipe(would have to have a new one constructed to fit single) go to waste.(of course turbos, intake,DP and intake pipes could be sold seperately)
Again, if I cant find a good price on single(I dont want to spend about $4000 more on my car(plus blow off valve is needed), $2000 is still somewhat steep, but, I want some more power.
Well, I figure since I posted in this forum, I know what you all will suggest and If I get a good price, then yes I would. Then istjust becomes a decision between the two kits(or can my car handle the R kit, if it was only kit avail at a good price). Or again, are there other quality kits that can keep airpump?
Sorry for the length, but it is an important question for me,
Thanks,
Steve

This is for a 2nd car to drive for fun, not racing(though it could participate in future).