I've got a cheap Jeep build myself.
Its a 97, bought 2 yrs ago for $3600, 113k miles, auto, basic interior, ripped drivers windows. Some may say that what I got in it isn't cheap. But I consider cheap in buying used and cheaper than new, swapping in better hardware for broken or weak link hardware (cheaper than repair and replace costs), and that I'm on a long term build process so money is not outlayed in a short period of time.
Maintenance is a big thing to keep up on a used Jeep. You buy used for a purpose and goal. You really don't know how well the previous owner drove and took care of the Jeep. So surprises can happen.
For axles, yah, some say the D35 is a weak link. Any money into it is a waste. Take the money and invest into a heavier accepted axle like the 8.8 or another stock Jeep axle (XJ's 8.25, for example). But if its not necessary to do now, leave it alone. Invest in your Jeep where you need it now and in short time. Long term concerns can be thought of and planned into the mods for today.
I started with a stock TJ. Maintenance necessary: shocks, steering stabilizer, 2 tie rods, front track bar, body rust, top, side windows, door windows, radiator, battery, and basic engine tuneup, filters, belts, etc. Oh, and front u-joints.
First thing mod'd the suspension with coil spacers, added adj. control arms to help adjust the axles for better angles (lower front/upper rear), added front adj track bar to recenter the axle under the Jeep, added spacer to raise rear track bar, gas shocks for 4" lift, bump stop risers to protect the shocks at full compression and body from the tires, upgrade the steering, gears/lockers/joints, installed 34" tires/wheels and wheeled it that way for a couple of months. Added front sway disconnects and removed the rear totally, added 1" body lift for better tire clearance, 1" engine lift as I replaced the broken trans mount, added t-case shifter bracket, and installed a winch I've had since 1992. All these non-seen additions help for the long term installs which come later. Finally added the last of the adj control arms so that's done. Swapped in a 8.8 couple months back after investing gear/locker into a D35. So I had to do that over again. Lesson learned. Recently added diff covers that were on sale. Bumpers were added recently.
Coming plans (and not in any particular order): family cage, rocker skids, flat fender (w/ bubble for 36" tires), rear flare to match front, flat belly engine skid, low doubler, gas tank skid, rock lights, lined interior, rear tire swing carrier, cooler/lunch box carrier system, rear corner skids,