First off, the PRIMARY reason you would incorporate would be to limit your personal liability.
Without knowing the details but given what you said you're working on, (performing reliability testing on motors, electro-mechanical systems, etc.) then I would
definitely incorporate (or LLC). If this item fails in use after you've 'passed' it, what are the potential repercussions? Is there even a remote chance that people could get hurt? You could be sued and end up in a world of shit. Let your Inc. deal with it and shield your personal exposure. Based on what you've told us I would say Do not do a sole proprietorship.
You and I have different definitions of 'small business' than the government does. I'm about 95% certain that you can forget about getting a $15,000 loan from the government. Simply not worth the hassle for the small amount you're talking about (this is considered a 'microloan' to them) and they're just going to refer you to a normal lender anyway (your bank or credit union).
If at all possible...DON'T QUIT YOUR DAY JOB.
The best way I would recommend starting this thing up is to start doing it part-time while working your day job. That way you have a regular income stream, you can 'consult' on evenings and weekends and you won't need to start hustling clients right off the bat just to put beans on the table. If you don't do this you'll probably start to panic once 2 weeks go by with no new work lined up and the current contract is running out. You don't need that pressure as a startup.
Start working up and sending out proposals on evenings to see if you get any bites, schedule around your existing work schedule.
Get used to the idea that you'll be working 2 jobs for 6-9 months.
I would STRONGLY recommend just saving up the money first to buy equipment.
MAKE THAT CRAP PAY FOR ITSELF!!!
Do you REALLY need to spend $15k?
Be honest with yourself. If you're only using it to buy a new F150 dually just to haul around a 500 lb motor once a week then you probably don't.
Can you GET BY with spending $1500 or $5000 now for used test gear then buying nicer stuff as you get income coming in?
I know SOOOO many people who go out and buy nice new tools, spend $$$ on specialty equipment then 12 months later they're selling it in a garage sale for 10 cents on the dollar.
Because they went broke trying to get everything they thought they needed at the start.
Sounds like you're going to be a consultant (of sorts) so you'll be selling yourself. If you're dealing with a bunch of auto-mechanics then you probably don't need to worry about your image (spelling and presentation) but if you're selling to a pharma plant then consider the presentation and image being presented in your written communication, quotes, bids, etc.
Does this help?