- If you want to do a conversion for cheap, I wouldn't attempt a 13B conversion. 4AGE will always be cheaper than 13B
- You can do a 4AGE swap for much less than $10 000, if you do all the work yourself you could probably do it for $4000
- You're right, 16 valve 4AGEs are old and nothing special, however 20 valve 4AGEs are a fair bit newer and a lot more powerful in stock form
- If you're not worried about costs, and you have the skills or you know people with the skills, then do the 13B swap. I'd say it's not a very common conversion in KE70s because everything will need to be custom/upgraded, whereas a 4age will basically bolt in with the right parts
- If you bought a wrecked RX8 there's not really all that much you could use from it in a KE70. There is the engine and gearbox, but everything else would need to be custom: custom engine crossmember/mounts, custom gearbox crossmember, custom tail shaft, upgraded diff/custom diff mounts possibly, upgraded struts with upgrade brakes, custom cooling system, new exhaust system, upgraded fuel system, hopefully you don't have to mod the steering to clear the engine or gearbox. Not trying to put you off it, but there is so much work and money involved, you would basically be modifying or changing every single part of the car other than the shell and interior.