gfunkadunk

Hi there, new here and new to Chevys. I am familiar with Fiats and Lancias, and had a F-150 for a long time, but just learning GM stuff.
I just picked up a beater 91 Vandura 1500 with the 4.3 engine. It doesn't start. The young guy I bought it off said it blew a head gasket and he changed it along with new manifold gaskets. He'd never done anything like that before, didn't skim the heads, not sure how clean or diligent he was in the process. After he got it together a mobile mechanic came by and installed a new diz cap, plugs and wires, but couldn't get it started.
I just got into it a little, was hoping it was just ignition timing but its more than that. I'm getting spark, sprayed some starting fluid into the throttle body, checked the timing, all i could get was a little backfiring thru the throttle body. Checked compression on the drivers side and was getting zero reading on cylinders 1, 3, and 5. Didn't look at 2, 4 or 6.
I think he put the head back on without getting cam timing right and bent some valves. And or it has a broken timing chain.
These heads look pretty simple to remove, I guess that's where I start?
While I'm at it, I know I'm going to want to improve performance, so what are the go to mods? What camshaft and intake should I look at and is the throttle body worth changing too? Not looking to change the pistons.
Thanks so much!
Looking forward to learning a new engine.