To start with I wouldn't buy a 88-91 Civic unless it's a Si. Look for a 90-91. You see those more than 88-89 at least where I live. If you want to remove back seat you can if you choose. I think you should have no problems keeping the seat in.
There's alot of motor options to choose. Definately remove the sohc 108 hp motor. These some motors you can use. :ZC Dohc 130 hp, B18A 130 hp cable driven tranny, B16A Dohc Vtec 160, B18c1 US GSR 170/JDM B18C 180 hp, B18C5 US Integra Type R 195 hp/JDM Type R 210 hp, H22 190 or 200 hp Prelude. Keep in mind, you'll need motor mounts etc.
Here's some sites you can research on prices etc.
Good Luck JDMCARS.Com, PasswordJdm.Com, JDMShit.Com.