Hey,
ive been a field tech for two crane manufacturers for 5 years now, and about 8 years in a shop before that working on semi's, garbage trucks etc. I dont have a ton of experience but here are my thoughts on field service trucks. Ive had a '13 f750 with a stellar body, and now a '17 f750 with a palfinger body. Unless you absolutely need a crane, why not go with a box truck like an isuzu npr? You can find an npr hd for pretty cheap, and they have a 14,500 gvwr - should be good enough for a generator, compressor, welder, torches, tools etc etc. You can find the lower box truck bodies that are really easy to get in and out of. The issue I have with standard service body is the amount of usable room you loose. The big service truck I have - I bet I would have more room in a 14' box truck for storage.
I do understand you have to start somewhere though. Just my two cents.
Good luck.