I guess the answers to these questions depend on you. What does a "mobile welding repair company" do, in your mind? Who are your customers? What does the "repair" mean in your description? Are you doing mechanical repairs? Or just repairing cracked and broken welds?
I will tell you this much.
1) I would not go into a partnership, period. If I couldn't hack it on my own and employ my friend, I'd let my friend own the business and I'd be the employee. There are enough blurred lines and question marks when starting a business without ownership and decision-making rights being involved.
2) I have owned SA-200's. They are fine for specialized work that involves just welding. They fall down when you need very much generating capacity, or when you start adding up the fuel bill, or when you start adding up the unnecessary weight for the functionality involved. I have not owned any SA-200's for several years now. I have not missed them.
3) I have owned several Trailblazers, with two of them on my trucks presently. The spectrum of work I do involves onsite ag repair, new equipment assembly, construction equipment repair, one-off equipment modifications and fabrication, and general commercial and industrial equipment fabrication and repair. I have not wished for anything more than the Trailblazers have provided. The TB325 EFI with Excel power that I own is by far my favorite welder. I only keep the older 302 out of stubbornness. Not that it does anything wrong, but I hate to hear that engine screaming just to power a 4.5" grinder or a halogen worklight set. The 325 does all of this at idle, not to mention welds at low RPM's also.
4) As far as truck sizes go...there are days that I get by OK with the F350 with 8' utility bed on it, and there are days that the International doesn't have enough deck space and tool storage area for what I need to carry. I would not consider a 1/2 ton pickup for this job. If that was all I had, I'd sell it and buy an old 1-ton dually to get started with. 2WD or 4WD wouldn't matter near as much as having the weight carrying capacity. Most places that are worth going to, will have the equipment to pull you out (or in) if you get stuck on their property or jobsite. Gas or diesel engine....I went with a gas engine for the F350 and haven't regretted it a bit. Diesels are a status symbol for 95 percent of the people who own them, and status symbols are always more expensive than necessary. It's gotten to the point that when I see somebody driving around in a diesel pickup, I automatically assume they have more money (or debt) than sense, until and unless they prove otherwise.
5) As far as building a custom bed goes, that's up to you. I have modified factory beds to do what I want. I would certainly not go and build something I could just as easily buy. Once you hang your shingle out there, you need to be ready to go to work, not putzing around reinventing the wheel. If you have so little work that you have time to build your bed in daylight hours, you would be better served by using that time to knock on doors in my opinion.
6) Lastly, I feel like what I'm about to say gets beat to death on every thread like this, but it just has to be said. Knowing how to weld or repair equipment is by far the easiest part of owning your own business. Any mechanic or welder can do that, and there are tens if not hundreds of thousands out there doing it every day. Managing your finances, meeting and keeping customers over the long run, learning how to properly estimate a job, learning how to sell your strengths and mitigate your weaknesses (everybody has some of each), getting all the paperwork right including taxes and insurances, etc....those are the hard parts. It can be done, but it is a long row to hoe as they say.
I will have been in business for myself 10 years as of next April, and it has been a wild ride. Only the last two years have seen me noticeably making more right decisions than wrong and beginning to reap some benefits from all of that labor. I guess you could say that I've devoted the best years of my life to the business, and it has come first over almost everything except for my marriage. I haven't worked a standard 7 a.m. to 5 p.m. workday since I can remember. Most weekends are spent on equipment maintenance and paperwork. I don't mean for it to sound all bad, but I do want you to understand that if you own your own business you are always working. Even if you're on vacation with your family, your mind is back home on what has to be done to keep that business afloat and moving ahead. It's difficult if not impossible for me to disconnect from my business, as I expect it is for any business person who really wants to succeed.