The swap can pretty easily be done. I know of a couple B3's with both 1.8t's installed. You will have to make mount arms for it though. If you don't want to make them yourself, I can put you in touch with someone who makes very nice stuff for these cars.
5 lug is pretty easy. You just need 4 B5 rear flanges. The brakes, you will have to sort out yourself.
As for the suspension, that's a little harder. Of course you have your normal springs. H&R, B&G, and weitec. There is also the K&W coilover kit. For that, I think you are looking around $1800, I could be off on the price... There is also the DIY coilover setup. A lot cheaper, but more labor intensive. If you go to motorgeek.com or theprojectpad.com you can look up this setup.
As for Interchangeable parts... The 89-92 80/90 are the same car. Be carful though, the late 90 came with the 20v, and bigger front brakes and knuckles. As for the CQ, they are generally the same, but because the rear weighs more, the suspension is stiffer.
I'm currently running a complete weitec suspension kit for a CQ on my 91 80q, and love it. It's a 45mm drop on the coupe, but on the sedan the rear only dropped about 35mm. Really helped me from scrubbing my tires like I did with the B&G springs I had on previously. Although, it did look a little better with the B&G springs...