220 volts is a given since any 110 compressor you are likely to find would be woefully inadequate for a sandblaster. By far the most important consideration is CFM because if it is low (below 15 CFM in this case) then nothing else will make up for it. A two stage compressor is much better than a single of the same HP rating and for a given HP the two stage will produce more CFM, cooler air and run quieter. In most cases HP numbers can be quite exaggerated so look at the AMP rating on the motor data plate to determine the actual motor power, at least 21 AMPs for a five HP motor (a 15 AMP rating is common and is only about 3 1/2 HP regardless of what the manufacturer claims). In any case CFM is what is going to make the difference in whether the compressor is adequate or not so make that the first consideration.
Also contrary to popular belief tank size will have next to nothing to do with how well the compressor will keep up and it absolutely will NOT make up for a lack of CFM! So look for CFM first and resist the urge to be drawn to the biggest tank in the store, you simply will not be able to see any appreciable difference in the way a 60 gallon performs vs an 80 gallon. The larger tank is desirable on a compressor of that size because it will have slightly less wear and power consumption over a long period of time due to fewer high load start-up cycles but run time vs waiting on air recovery time will be the same for a 60 or 80 gallon, the extra 20 gallons will gain only a few seconds of extra run time with a sandblaster and then even that will be lost to the longer recharge time so it is simply a trade off and you will gain exactly nothing. The bottom line is get the larger tank if it don't cost more but don't spend much money to get it and whatever else you do DON'T pass up a compressor with more CFM to get one with a bigger tank!
Remember CFM is what runs your tools and a bigger tank does not make a bigger compressor!