My latest purchase (17.65mm) was an assumption.
When doing an assumption most Fannie lenders will allow up to two supplemental loans in addition to the original but won't typically exceed 75% LTV for the combined loans. The supplemental loan will have a different interest rate but will be coterminous with the first (same maturity date), and will have no replacement reserve escrow because the original loan is already making those contributions. Caveat, I've seen a friend apply for a supplemental on a project he held for 2 years and had added tremendous value and had taken no supplemental and the lender said NO so he ended up selling instead. There is nothing in loan docs that will say you have the right to X number of supplementals so the lender can indeed decide they don't want to add more debt to the project.
If there is any IO period remaining you will get that as well but only for the original loan, the supplemental will not any rehab $$ or IO. If the seller has already used one of the supplementals then once you close, if you take out another, you will have no ability to take another. A cash-out refi is usually only doable with about 6 months remaining on the loans so that can restrict your exit strategy options because if you now want to sell the potential buyer can not get additional loan dollars and if they put fresh debt on it there will be a very large defeasance (prepayment) payment you will have to come up with.
Final thing, don't just assume you can get the assumption approved by sellers lender and don't go on the word of anyone except the actual DUS lender. The lender will look at the buying entity and weigh them against the current owner. If you don't have the same balance sheet or experience behind you then you are deemed to be riskier no matter how many times you can cover the loan balance in networth. If you pass your feasibility cutoff and aren't protected with a good finance contingency your earnest money is at risk which can be as much as 1% of the purchase price.
Little more to it than fresh debt but it can be worth it if you get a good price and the interest rate works.
Just know what you are getting yourself into but it can be a good thing.