I am building a set of wheels for my 1970's road bike. I already purchased some 36 hole hubs and am now trying to find a set of decent rims to build the wheels. I probably won't be taking the bike in any cyclcross races anytime soon but I would like to have some wider tires so I can take the bike on some offroad trails. I am thinking of putting some 700 x 32 or 700 x 35 tires on the new rims. According to this chart (https://www.sheldonbrown.com/tire_sizing.html#width ) it seems I would want an interior rim width of about 16 to 20 mm?? Ideally I would like some rims and tires so I can still use the bike for commuting on the road with out losing too much speed but be able to take the bike offroading as well. Any suggestions for some reasonably priced sturdy rims?

SUN CR-18 rims are well-regarded for that sort of thing.

Mavic Open Pro can handle a tire that is 35mm. Salsa Delgado too.

Since you're talking about road riding, you aren't looking to build some monster cross animal. Any normal rim will take up to a 35 or 38 and it's unlikely you'll get more than a 32 or 34 into a road frame. ATMO, Look for bargains or whatever else appeals to you.

i'll second the CR-18 recommendation.