Repairing currently isn't that much of a time drain if you have a decent nailgun. The issue is getting a decent nailgun. Frankly, the main time cost is just moving from one block to the next if they've damaged a whole bunch of things. Maybe make a recipe for nail gun parts? Mechanical parts, plastic, and oil? With a decent nailgun, repairs aren't really an issue.
I am kinda iffy about the idea of replacing an existing block being a thing -instead of- repairs as they currently exist. I like being able to flip over to the nailgun or hammer between waves and quickly repair all the damaged blocks. I wouldn't want that to always cost 2/3 of the entire block cost.
Also, I will be a little saddened if your single player version ends up a lot weaker than the current. I am really enjoying the challenge. Just completed night 3 on survivalist, have steel already, 2 crucibles going, have a q190 nailgun, a shotty, and a solid supply of exploding bolts... best start yet.

My concern remains that changing difficulty has no effect on horde size due to you using heatmap hordes instead of GS dependent ones, so any rebalance you do can't be tweaked with difficulty settings.
Other note that comes to mind is, watching Like A Boss play your latest experimental build: The costs of everything have been upped, but the rewards for killing haven't...? This risks a situation where each kill costs more money than they give. At current, the 200c/zed easily recoops any reasonable cost of killing anything other than cops/wights/radiateds, with some to spare for building materials/food/health/repair kits/etc. You always feel like fighting puts you ahead of the game. Change the costs of things too much and this starts to become questionable, at least until you have the capacity to make your own steady supply of ammo. Noticing the high cost of weapons in particular was distressing, given that your starting weapons are random and there is a HUGE gulf in efficacy of weapons for dealing with a horde. If the cost of a Deagle or Shotty is 18k or so, that will put it out of the range of things one can reasonably buy day 1. Which effectively means that a random start that gives, like, a 9mm pistol, a hunting rifle, an smg and a bow is an instant restart. Basically any start that doesn't include at least a shotgun or Deagle, just due to the extreme differences in efficacy and cost efficiency between weapons(for some players/builds, sniper could also be counted, but it's more specialized).