Good topic. One goal I'm working on this week is writing up an investment proposal for friends/family that would be interested in private lending. I've dealt with hard money for a few years now and think it's time to go for private money!
As for the legal, rules, guidelines side of things, I believe the only thing you need to do is have a solid formal agreement in place that would be used to loan money and/or record a mortgage/lien on a specific piece of property. I'd say make sure the agreements outline EVERYTHING. The standard items like loan amount, interest, timing, the property, late fees/penalties, etc. But also stuff like insurance coverage and what happens if the house burns down half way through renovation!
And yes, it would just be considered interest income, placed on the 1040. Also, say you're doing a 6month loan in August, you could have in the agreement that interest is paid upon principal payoff. So interest and principal ends up being fully paid to you in January and you could defer that interest income until the following year.