I haven't used one yet, but these allow you to borrow construction costs.

Main similarity is that all 3 allow the borrower to include rehab money in to the loan.I would recommend using contractors that have earned the accreditation as a Certified 203k Contractor.

When you spread fixed costs over a larger loan amount, it becomes more cost effective and the Lender is sharing risk with the borrower (vs. an all cash deal).  

The Solo 401(k) also has the advantage of being more favorable for real estate investments using debt-financing such as a mortgage - as the 401(k) is exempted from a small tax called UDFI that an IRA would pay on the percentage of income derived from the borrowed money.So, as you continue your research and get feedback here on BP, think about what type of program will best suit your needs and be sure to ask questions along that line.

Would I be able to talk to a lender to get an idea of what I can borrow without having a particular property in mind?

We qualify borrowers by DSCR (Debt Service Credit Ratios).

When it comes to private lending companies, the interest rates for cash out refis are starting somewhere around 9% for borrowers with good credit right now. 

that is 80% of the equity. you can only borrow against what you own, otherwise the bank is double lending you money for the same asset. but thats still $140k in cash. which would be an amazing amount for a self financed brrrr, or downpayments on brrrr loans for multiple brrrrs, or you could purchase 3 $200k MFH with 20% down and have 6 doors right off the bat. lots of choices.

I’m sure I could always borrow as much as they’d allow and then just use what I need and keep unused amount and return it.

The amount you can borrow may vary from state to state but FHA is a federal program so a lot of the requirements are the same.