1) Agree with prior post that the cheapest glider isn't going to be the best. I spent $400 on my MR2 but another $2000 or so on just chassis repairs (brakes, paint, etc) If I had been more patient I could probably have gotten a good chassis for less than what I spent rehabbing it, and certainly also saved time in the process
2) Use lithium. It wasn't economically feasible for me when I built my car, but it is now and so unless low initial cost is your only concern there is no reason to be going with a lead acid battery pack today.
3) Insulate your battery pack, or at least leave enough room on the top, sides, and bottom such that it is possible to retrofit insulation as needed. This may be less of an issue with lithiums but they are still susceptible to cold temperatures and even 1/2 inch of builders foam insulation will help substantially to retain heat.
4) Don't scrimp on wire sizes. We all know not to undersize the main battery and motor cables, but using smaller size wiring for secondary circuits such as for the charger and other loads may lead to needing to replace the wiring if for example you go to a more powerful charger in the future. For example I originally ran 20A rated extension cord from my charger's output to the battery pack which was fine while I had a russco charger, but when I upgraded to a manzanita I had to re-run all that cabling and upgrade a bunch of fuses and terminal blocks. Depending on the car and the layout of components this may not be a big deal, or it could be a major pain.
4) Leave room for upgrades. This goes for all major components. Your current charger/controller/whatever may fit perfectly in that nook/cranny under the hood but the replacement / upgrade one a few years down the road may have to go under the passenger seat or force major rearrangement of other components if you didn't leave enough room. Of course if you have a small chassis and a big battery like I do, you may have to make compromises.
5) Leave maintenance access. I have to put my car on ramps to get at the motor controller since it is mounted low in the car which is a bit of a pain. If you can place things where they are easier to get at, it makes maintenance much faster, cleaner, and easier. Of course again if you have small car and big battery like I do you may have to compromise here.
6) Favor weight over aerodynamics. I computed for maximum efficiency at freeway speeds when I was designing my car and selecting a chassis. However typical EV conversion usage has a lot more local stop and go type driving where a reduction in overall weight is going to be more beneficial than a more aerodynamic chassis. Note that using lithium batteries would be a major improvement but even then a sleek 2500lbs chassis is probably going to get worse wh/mile in an urban setting vs. a less-sleek but lighter 2000lbs chassis, especially if you have hills to deal with.
7) If you are not looking to build a high performance car, seriously consider one of the low-voltage (relatively speaking) HPGC/Curtis AC systems. For not much more than an equivalent sized DC system you get the Regenerative Braking
(especially bonus-ful if you have hills) and reliability of an AC system. Tradeoff is you will be limited to a 108v battery pack meaning bigger cabling and cells to deal with.