IMO, if you have apart you should do a full rebuild, especially if some of it is starting to fall apart/all gunked up. It'd really suck to do half of it just to have something that you didn't replace fail 3000 miles down the road. If you're doing just a stock rebuild it shouldn't be too awful. It's near impossible to say it's going to cost exactly $xxx because things come up and prices of parts change. My guess is to have the engine completely rebuilt with all new parts and reassembled would be about $2000. Mine was over $4000, but we did lots of non-stock "improvements."
Hope that helps

49 Chevy 3600
65 Chevy K10
66 International 1300 dually 4x4
46 Ford 1/2 ton
76 Tucker Sno-Cat 1642
96 Dodge 3500 Cummins dually flatbed x-cab 4x4
06 Jeep Wrangler LJ rock crawler
If you can't fix it with a hammer, it's an electrical problem