What will you be hauling for your business? Back when I had a station wagon, the back seats folded down and the front passenger seat folded foward, allowing me to move a comparable volume of stuff as a pickup truck bed, minus the open height those have. All my cameras, tripods, lighting stands, PA system, carts, cable bags, tool bags, etc fit in there. While that station wagon only made it to 238,000 miles, which I consider fairly young since I rack up road miles like crazy and drive cars into the ground, it had great fuel economy.
I recommend putting feelers out with your extended family, friends, and neighbors before hitting the "GP" shark tank. Like with jobs, anyone you have a remotely personal connection with almost always has a better deal for you than a stranger. Even if they aren't selling a car, they may be able to recommend you for a sweetheart deal with a place they know, or a friend of a friend. All of my cars have come from someone I know in one way or another, for cash. I currently drive a literal grandma sedan that she barely touched and I was able to grab for well below market value before her family was ready to list it.
Hit up your local FB groups, organizations, and buy/sell/trade groups, too. An otherwise stranger may be willing to give someone local a good deal as well. If you've put down strong roots in your community, they may pay dividends for exactly such a situation. Likewise, if the car you end up with has a few things wrong with it, your local relationships could get those addressed for free.
As for business accounts, Amex is super lenient and probably the best to start with. People often get approved for a Blue Business Cash/Plus with a young business and no real revenue yet, which is a nice card to have if you're worried about utilization showing on your profile. That card does not report to either business or personal credit, making it a "stealth" tradeline that can you spend what you need to on and no score dings.
Strategy-wise, I don't like holding checking accounts at the same institutions that I hold credit cards with, at least not your primary checking, and not in the early stage of a business. If you keep them jealous and in the dark about what you have and where, they may give you better offers/approvals. There are nice little side effects from this, too - since I paid my personal Amex from a business checking account elsewhere, they knew I own a business, and since I paid decent sized statement balances, they knew said business has cash flow, but they could only guess how much (they usually guess higher than you actually have if your credit habits show the habits of more financially-savvy people). They constantly sent me offers for the BBC/P card and when it did come time to grab a BBP, it was an approval with no proof of anything about the business needed. They already knew what they thought they knew, so the rest was a formality. If you apply without "priming" the lender about your business first, they may put you under a lot more scrutiny or not approve.
Part of building a strong profile is just window-dressing, making yourself look more desireable and stable than you actually are. I'm around your age, think of it like making an online dating profile, one you can only edit at a glacial pace. You want to make lenders swipe right on you. If you're not rich, adopt the credit habits of rich people (pay full statement balances, don't micro-manage utilization, no BNPL, etc), if you'd love to get more credit, don't act like it (age your accounts, use your inquiries judiciously), etc. Lenders want to give credit to people with high income and who don't need credit.
You don't need the AU accounts. Lenders just disregard them in lending decisions since you're not legally obligated to pay those accounts. The score boost you can get from AU accounts is useless fluff. You can basically go for what you want in your own name at this point, you just need to let your existing cards age more.
A big part of profile-building is getting CLIs and building limits high enough that your natural spend yields low utilization on its own. Premium cards often require you to qualify for a $5K or $10K starting limit - lenders want to see how you've managed your existing $5K/10K cards first. Oops. Too many people apply for those without priming their profile. In many states, a low/ish average credit limit is also a ding on credit-based insurance scores. To get that ball rolling faster, NFCU and Amex tend to be very generous with limits. For others, you may need to show high utilization. If you are showing 1%, why would they give you a CLI? Course at application time, you want to be showing that 1%, but when not applying for credit, let your true spend show. The score ding is temporary and meaningless since the goal is to induce a CLI. Discover can be very stingy at first, so you will need to put heavier util on that thing if you want it to grow.
As others said, if you do get an auto loan, you don't need the SSL. I would decide whether you are getting a loan or paying cash for a car before doing the SSL. One open installment loan satisfies the credit mix portion of a score and an SSL paid down to 9% for the score hack is fluff from a profile strength perspective. An auto loan is much meatier.
VS4: 709