The Catera can be a good car, but it will take A LOT of work to get it there. The car is on the same chassis as the GTO, so that helps things, but you will need to be able to fabricate if you plan on using this chassis. It's basically a rebadged Opel, which is where the engine came from.
Problems with the car:
engine failures
wiring problems
front suspension/won't stay in alignment
16.5" wheels make it difficult to find tires
transmission is nothing special
upsides:
you can find them with blown engines for $1,000
good platform
it's still a caddy, so comfort and a nice interior
A good way to start fixing GM of Europe's mistakes is to go with either an aftermarket front k-member, or fab up a tubular one. Assuming you get the geometry correct, this will take care of the alignment/suspension issues and is where the motor mounts are placed. An LT1 is a cheap and solid motor that will fit with ease into that chassis. I *think* that there were some Catera's that came with a manual tranny, but they were rare. If you find one, it would make it much easier to swap in the 6 speed too.
Doing all of that will take care of the majority of the problems with the car. Using the LT1 harness will clean up a good bit of the wiring problems. It's a lot of work to get one of these cars to be worth having, but it would be damn fun when finished.