Looking for advice. Want to build a Duster to run on a budget.

I'm not totally sure what people are saying that 4k is not enough. I put a v8 in my car for under $1500 in 2021. I found the engine for $200, swap kit from trans dapt was $90.00, I just had manifolds on it for starters, no headers. (unnecessary cost for a cruiser), a chinese air gap is sub $200 depending on the brand. the 7 1/4 rear end is still in my car and it hasn't blow'd up yet. the driveshaft from the slant worked. I went through 2 transmissions over 3 years and each one was $150. Rebuild the drums, new master cylinder, and rubber hoses and it'll be safe enough to drive. Grease everything possible and run it. (I ran mine on the original suspension for 2 years and it was totally fine.)

You don't need any electronic ignition, you don't need a new wiring harness, (unless its gone, but get a cheapy one from amazon and figure it out), you don't need new upholstery just throw some seat covers on it. my carpet was turf mat from home depot for $50.

My only advice is to use a 904 because it'll drop in without any worries and just use 3.23 gears or 2.76 gears for highway use.