One of the alternatives has been in Qld for a long time. QR and Lakeside run under them. So far, 3 rallysprints have been run under them with another rally sprint and a proper rally to run under them this year alone (and maybe a motorkhana or two as well). :)
To the OP, if you have the funds and the ability to build your own, then do it. But buying a prebuilt one will be cheaper though. The down side is you are also buying whatever stuff ups/ short cuts/ last minute fixes that the previous owner has made :) .
I'd avoid Geminis or 1600's cause good panels are getting nigh impossible to get. If you want rwd, then might I suggest an NA Silvia, then turbo it once you have gotten everything out of the car that you possible can.
As far as ffwd's go, there is a Proton for sale on the Brindabella forums which is priced very competitively against the excels, or be different and build something original like an Echo if you want to stay faithful to Toyota. :)