On the investor front it depends on the debt size. Most seed-stage investors would prefer debt to a competing investment amount. Loaning yourself up to a year of lean run-rate is probably fine, but much more (hiring, paid user acq, etc) is going to look weird. Future investors may ask you to write off the loan entirely if the terms are nuts (compounding or high interest rate), but shouldn't bat an eye otherwise.
Regarding contingency plans, you don't need to have a hard repayment date in the loan terms and can let it accrue interest indefinitely (until bankruptcy, acquisition, or otherwise). Unlike traditional convertible debt a founder loan normally doesn't "blow up" into a huge equity stake if not paid back.
If things don't work out and you have the opportunity to roll the founding team into an another company (acquihire), loans are an easy thing to assign value to, even if the IP or goodwill is more difficult. Negotiate the loan repayment as a signing bonus if you can.
If things work out and you either raise money or bootstrap to profitability, you can pay off the loans as it makes sense, or just forgive them outright if that's easier. Either way you probably don't need to involve your whole board to manage it, unlike equity changes.
I get the desire as a founder to obtain the same terms on capital as future investors, but it can put you in a weird place and can complicate future fundraising. Props to anyone who can make it work, but I had good luck with the founder loan process and felt like it was the cheapest way to finish bootstrapping (we did).
Good luck either way!