I have wire brushed my work for years, and I do absolutely HATE wire wheeling viney twisty stuff ( and that is the bulk of my forge work) I briefly tried pickling, but dont like the chemicals involved and had rust issues when things weren't neutralized properly. I would love to have a shot blast machine, but the cost and space arent really practical, so I am looking at a tumbler. I have this old condemned 100 pound propane cylinder, that looks like a good start, and the remains of my old crane, a whole lot of pillow block bearings, but all my motors seem a tad large. I have a 1.5 hp that would be ok I guess, the other candidate is a 1/2 hp that is a 3600 rpm. I dont really want to have to slow things down that much. Has anybody here built one of these? What kind of speed/hp would you recommend? 

Slow!  Too fast and they don't tumble. I've scrounged a speed reducer for when I get to work on mine. (Found an old commercial large roll up door motor/reducer/sprocket I want to try as well. Got about 4/5 of a bucket of used SDST metal screws to try as medium too.)

I wish I had scrounged a conveyor drive from the cookie plant. They were usually toast when they got replaced though. I was thinking 1/2-3/4 hp at around 60-80 rpm at the drum. I have a few 1/4 hp, then jump up to 1.5 and bigger and that seems a tad more torque than needed. 

TP beat me to it.  You want a tumbling action and too much RPM gets in the way of that--way too much and the stuff basically sticks to the outside via centrifugal force and a bit too much and the media and parts are being thrown rather than truly tumbling--sometimes resulting in damage if parts whack each other in the process.

On the other end of the scale, too little rpm and the media/parts slide down the side and you get flat spots but no real cleaning action.

A nice rolling "wave" is what you want.

The actual speed depends on lots of outside factors including just how full the chamber is but my guess is that you are looking at somewhere between 45 and 60 RPM based on what I see there...and "guess" is the operative word here.  That implies your 1/2 horse should normally be plenty once it's geared down.

Our tumbler has an octagonal drum about 24" in diameter and runs at about 60 rpm with fine ceramic polishing media.  Parts aren't delicate at all so can take a bit of "throwing" and fine media tends to cushion a bit better than coarse.

Noise is always a problem..they just aint quite so you might consider some minor ways to help with that--cushioning in the right places for instance.  I normally use a large vibratory tumbler in my personal shop and the noise of that makes it downright unpleasant to be around.