@Brandt Miller seems like you need a partner. Here’s a scenario:
Team up with your “trusted contractor” - you front the money to buy the deal. He fronts the rehab costs. You offer $400k with 25% down, seller financed at 8%, and your contractor partner does the rehab at risk. You and your partner agree to use sale proceeds first to cover costs, then split all profits after that. With this partnership structure, he has an incentive to finish the job quickly and minimize rehab costs (minimizes his exposure), there is extra upside potential for him in the deal beyond just the contracted work, and you don’t have to front any money for the rehab, just the $100k to buy the deal and the note payments until you cash out refi. If he’s on the fence about partnering you could even sweeten the pot by offering to split a share of the post-refi rents with him, basically a royalty.
Using the numbers from your example, you buy at $400k, with $100k down. Your partner puts in $200k worth of rehab labor and materials. You refi the property for $800k (worst case?) at 75% LTV. You both get all your initial investment/costs covered by the refi payout, then maybe split the rents 50/50. If he's able to do the rehab for $150k, let's say, then you both make an extra $25k on the refi payout. And if he goes over and rehab costs $250k, you split the "loss", and each leave $25k in the deal. Best case your ARV is at the high end and rehab budget is right on target, and you refi at $900k valuation, the. You both make an extra $50k on the refi for the same amount of work.
Just make sure you know your market well before you buy, and that you really trust your partner to execute well. Good luck!