Mel, I would say a lot of it would depend on your budget. Artograph has an amazing new product, very small and portable, but it's about $700. I don't care for the inexpensive opaque projectors because they get so incredibly hot and will ruin a photo if you don't use a copy.
But there's a lot in between. I prefer something that you can use with a laptop like the business presentation projectors you mentioned. You can find lots of those used on Craigslist or eBay. Just make sure the bulb is good because they cost a lot to replace.
One inexpensive option is the old overhead projectors which you can often find in thrift stores. You have to print out the reference photo onto a transparency sheet (which you can get an an office supply store).
John Mayatt uses one in this video of his "Fame in the Frame" show (4:40):