@Cami Bohn Congratulations on renovating your home and adding that much value! That's amazing!!
If the buyer is financing the home and it appraises for less than your agreed upon offer price, then either the buyer has to come out of pocket for the difference or you can choose to lower the price. Do you really think it'll appraise that much lower? Sorry I know it's been 9 days since you posted this so did you accept the offer and move forward already?
I use Carrington as my lender. I'm not sure if they work in California but let me know if you're interested and I can send you my contact's info. Most lenders will do investment properties. I advise you to shop around at least a few places to get the best rates and as little fees as possible! We got Carrington's rates and fees to come down because we got a quote from Ally Bank and Carrington beat it. So don't be shy about shopping around.
I have not done a rehab split before but this is what I have done. We partnered with an investor I met on BiggerPockets from Oregon. He wanted to get into real estate but didn't have the time or knowledge of the Jacksonville market. We had the time and knowledge but not as much cash. He came in with a larger % of acquisition costs than we did. We did 70/30. Then for ongoing maintenance issues, cash flow and equity we did 50/50. We found the deal, analyzed it and I manage it for us at no extra cost to the partnership. You can do something similar where your brother in law puts more money down to acquire the property or all the money down if you are paying for the renovations. There are endless things you can do here! Just propose some options and you can all talk about it. Make sure to create an operating agreement, even with family. You can draft one yourself using a template you can find online.
As for the ideal first property, it depends if you want a rental or a flip as you said. I think there is no perfect first deal. You just have to know what you want (rental for cash flow, appreciation, flip, brrr, etc) and find something that matches that goal. Then do your analysis, be confident and jump in! Don't overanalyze and be stuck in "analysis paralysis" as they call it here. But also make sure you're not overpaying for a property. You can start by analyzing properties you see online just as practice and ask yourself if it would be a good investment or not as part of your goals. Then when you are ready, go for it!
Hope this was helpful!