Here's what I do...
Production buildings: on one base, you can afford 3 rax (2 lab + 1 reactor) almost constantly producing + constantly producing scvs + getting some upgrades. (Expansion fits in too; it is faster/slower depending on when you're willing to cut other stuff to make it happen). On one base, your mining rate is somewhere around 950 minerals/minute, if you're getting those MULEs. The gas gather rate is about 4:1 (discounting the mule), so say 200 gas/minute. (Obviously I'm rounding here but it's good enough for the general idea). In m/g/secs, marines are 50/0/25 and marauders are 100/25/30; so w/3 rax at "steady state" this gives about: 600/100 (4 marines, 4 marauders) + 150 (3 scvs) + 200 (2 depot for the 15 supply added). (I say 'steady state' b/c I balance income/spending manually during the phase where I'm building structures still.) You're already building up gas, so 1) only use 1 geyser or 2) cut a unit or scv here or there and build up minerals & get upgrades and/or tech up.
Usually when I expand, I throw up another rax or two, a factory (tech lab), and a starport (i usually reactor this b/c my macro slips in battle). I like to have a little too much production capacity on 2 bases b/c I can get stuff going faster when I inevitably let my macro slip :/ And on 3 bases, if my army starts to get large, I'll throw down a bunch of production structures so that if I take losses, I can replace them quickly. At some point if you near the point where you can't spend money b/c you're close to the supply cap, it's very effective to build a lot of production buildings to help recover losses.
So w/the other structures on 1 base, you could imagine doing... 2 rax marine/marauder + 1 starport medivacs. That might run you something like 500/150 (units) + 150 (scv) + 200 (depot), leaving extra room for upgrades or expansions... or opening the possibility of adding in some hellion production or more rax units. Or maybe double hellion + medivacs + some marines to fill gaps. Or 2 rax + tanks.
So yeah I think '3 production structs per base' is a good rule of thumb. But you should balance it based on the unit composition you want, not just "oh i can only have 3 production structs."
rax tech lab upgrades: I'm 99% sure it's best to get stim first (well, unless you are forgetful and never use the stim, lol); source: day9. Marines/marauders are SO much more effective w/stim, especially in smallish numbers. Throw in a little bit of stop micro & you have something awesome there. Whether I get concussive or shields next depends on what the opponent is doing. (I usually move out w/stim so I'll see what's up by then.) Engineering & +1 attack is another possibility for the next upgrade choice, though I rarely get it that early (not sure if this is good or bad).
edit: if you're using marines & shields isn't your 2nd upgrade, then it should definitely be your 3rd upgrade. It's really useful... makes all the marines last like 30% longer! It's critical vs things that rape marines like colossi, siege tanks, banelings. It also synergizes really well with medivacs, since the longer the marine stays alive, the better the chance of getting some healing love.