Production EV battery packs almost always include some sort of BMS and have lots of interlocks and safeties. You don't really have to do much to work with them other than to talk to the BMS and not exceed voltage and current limits. If you do, the BMS will trip the pack's internal HV contactor.
Safely installing/modding/repairing automotive fuel systems is likely just as, if not more, dangerous if you don't know what you're doing. There are few interlocks on automotive fuel systems beyond "if the engine stops spinning, turn off the fuel pump" or in some vehicles, there might be a crash/rollover fuel shutoff switch.
People think working on EVs is super scary when reality is that you can usually pull a cover in the rear passenger area, find and pull a bright orange plug, and the HV system is rendered completely safe. EVs don't have a billion places you can scald or burn yourself, their "engine" compartments generally don't have much or any spinning objects that could deglove your hands, there's no fuel or oil to catch fire, no high pressure hydraulics...
Do any of those BMS do cooling? The parking lot at work hits 115F regularly and that is bad for batteries. I'd love a conversion, but I worry I'd get nissan leaf battery life vs tesla s battery life. I'm thinking something like an old mazda bongo import. For that matter, do any of the kits power the ac compressor?
Is there a place you know of to get production EV battery packs? I'm an amateur mechanic, and this might be up my alley, but (as above) I'm worried about these battery fire situations.
All true, but a pack that has been in a crash might be broken in subtle and hard to detect ways. A short in the pack would be a quick way to ruin your day.
I assume that at this point there are enough wrecks of Model S/X/3/Y that you should be able to pick up electric motors from them at junkyards.
The thing I really worry about with electric swaps is battery. You could have a Note 7 situation on your hands _real easy_ if you're not careful.
[1] https://performanceparts.ford.com/part/M-9000-MACHE