as 72blu noted, wheels are going to be your single biggest deciding factor. from there, it's where and how you want to spend your money.
I'd KISS and just upgrade to 73~76, several companies sell the full kit now so all you'd have to do is track down a K-member. granted you could stay with the early style K, they make compatible sway bars now but it's better if it's all the same system. easier parts compatibility if you're not doing a full on custom build.
from there you could add better pieces like tubular uppers, the big brakes (they have drilled & slotted rotors avail), good shocks, better pads, bigger sway bar, greaseable pins and then knock it all together with good quality suspension wear items.
out back, I'd buy new BBP axles and throw some discs at it.
on the t-bars and springs I tend to err on a little bit of the lighter side. I think 920's or 990's are more than adequate for a spirited street driver.
100% I'd throw in some subframe connectors.
swapping out to an OD transmission is an undertaking involving a fair amount of floor surgery. that alone would take up a good bit of your budget.
all said though, I think with some smart planning and judicious purchasing you could get everything done-- including the motor stuff within your budget.
