The main consideration is a decent vacuum. Most of the portable manufacturers use the same vacuums. You can buy a Mytee or a Ninja or one of our Olympus models and have a small portable that will extract small urine spots.
If what you want is really a small portable, buy a brand name from your local distributor. Don't look for an Internet special unless you plan to service it yourself and don't care about supporting your local distributor.
However, the cost of even a small portable will be $1000 or more. Even a portable has some time to carry into the job, set-up, fill with water, pack back to the van and so forth.
A better idea might be a simple spot machine. DurrMaid makes a good one if you want heat. If the main use is for small urine spots, you won't need added heat. I'd suggest a PowerX spotter from our Olympus line.
Scott Warrington