Engine choices
You may want to start with Vans.
http://vansaircraft.com/cgi-bin/store.cgi?ident=1431131973-248-297&browse=engines&product=lycoming2
Choices
Constant speed vs fixed pitch. CS must be specified when ordering and which governor.
Horizontal vs Vertical induction. One uses the traditional cowl. Other requires a modified cowl. Horiz is standard. I think.
Compression. Lower is less HP but also less wear. You can burn lower octane fuel. Higher is HP but requires higher octane. Overly simplified.
Parallel vs angle valve. Higher performance with angle at greater weight and cost
Injected vs carburated. FI is easier to balance injectors for LOP and inverted ops. Carb requires heat. Overly simplified. Also which fuel injection and if so which fuel pump and does it require return line plumbing.
Roller tappet vs non roller. Lycoming added the rollers to eliminate the wear point on the cam. On Lyc engines a cam or tappet problem requires a complete tear down for replacement. The jury is still out as to the success. Most engine builders don't use roller. Lyc rollers are standard.
Mag vs EFI. Traditional mags or PMags. Pmag is basically a mag with the capabilities of electronic ignition. EFI is more like auto ignition.
Spark plug bosses. You can specify auto plug bosses if you want to use auto plugs.
I'm getting edumicated but not quite there yet.
For now
IO360, low compression, parallel valve, p-mags, Airflow Performance FM-150 FI and pump, auto plug bosses