thats ok dude we all make mistakes,now lets get down to the matter.
1)any celica with an s in the engine number no matter year or spec(jdm,us,eu etc.)can do a swapp with any toyota engine with an s in the engine number without to much hassle.egs. 5sfe to 3sge or 3sgte etc.engine sits at an angle tilting backwards.
2)you started of with a celica engine that has "a" in it so it would be easier to replace it with an "a" type engine. egs. 4afe to 4age.motor drops right in, no refabricating of mounts and engine clearing cross-member problems.engine sits straight up.
3)the 5a is smaller i think(dont quote me on that) 1.5 liter
but it has slightly more horses because of the ecu and emmision control.
4)this is my thought,if you are going to do an upgrade,or change the motor,do it with something you can feel good about.it will be difficult to go from 4afe to 3sge or 3sgte or the least 5sfe(cause those motors sits at an angle and the mounts and crossmember would have to be changed!
the 4afe you said costs a good penny,and the 5af even though its an improvement isnt much to talk about!that leaves another angle,why dont you stick to the "a" motors but go with a high performance one.like the 4age 1.6 liter twin cam,(they came in the mr2,corolla gts/fx,levin,etc.)its a direct drop in.they came in a wide range of years and all of them has more power and torque than the 4af and 5af some are well over 170 hp with hard hitting torque to match!and can easily be upgraded with mods to make for a hi revving 1.6 liter beast! the decision rests with you however,i can only give you advice on what i think from a technical point of view.
if it was up to me i would,get my ass on ebay and search for a 4age with tranny and ecu,or even a 4agze(same motor but with supercharger )or even a 4age 20valve with tranny and ecu.the above should run you from 700.00 to 1200.00 and they all drop in with little or no modification except for harness routing and fuel pump etc.
btw for info:any toyota motor with "f" in its engine number usually mean economy,regular driving,and those with the "g" means hi performance,hard, hi reving driving. egs. 1zzfe and 2zzge,4afe and 4age,3sfe and 3sge. there is only one exception that i know of which is the 4efte the turbo starlet motor(somebody probably goofed on that one, put an "f" instaed of the "g"
let me know how you are going to go,but dont let nobody,me included make you do something you are not sure about and dont let those guys push a motor on you that wont improve drastically on what you have.