Opening and cleaning a MacBook Pro (retina 2012) I have had my MacBook Pro retina for more than 2 years, without cleaning the insides yet

I have used it extensively, many hours every day, and heavy tobacco smoking for a year or so (the fan outlet grills are yellowish)

Should I just order a screwdriver set (pentalobe?), open it and clean the fans etc.?

I assume warranty is already expired (no apple care here).. But will opening and cleaning it be complicated or risky business? I'm good with electronics, but Apple uses tiny delicate screws/parts etc.

Should I use a swab, compressed air container, or will a vacuum cleaner do? (metal/plastic head; static electricity)
Assuming the warranty is expired, you'll be fine. If you want to just remove the dust from the inside, you're on the right track. Get a can of air, a vacuum, and a pentalobe driver set. 

Don't actually vacuum the inside though - turn it on and hold the nozzle up to the outlet of the area you're blowing the dust from. Static won't be a problem as long as you don't touch the computer with the nozzle.