Can't say I've ever purchased one as such. Just used large aquariums. I'll be very interested to see what you go with.
Are you looking at complete units? (Ones that have lighting and a misting/watering system already installed?)
What size are you considering? Do keep in mind that they fill up quickly -- far quicker than most people expect. When it is sitting there empty, it can look enormous/spacious. As you add a plant here and a plant there -- not to mention as the plants grow -- that once "spacious" area can become cramped before you know it.
Also, do you plant on decorating or landscaping the orchidarium or leaving it very utilitarian? Landscaping can very quickly and easily eat up half your space. On the plus side, though, it tends to look infinitely more attractive than a utilitarian look.
That said, my first cautionary note: Plan ahead. If you already know which plants you plan on getting, determine how large each is likely to get -- both in height and diameter spread. (Obtaining a large box from an appliance store that you can cut down to size may help you visualize both the space the tank will take up as well as how much room each plant will occupy in that space.) It would be a major let down to get all excited over you new acquisition, only to quickly discover that the plants you want will quickly out grow it.
Next, make sure that all parts of any lighting and/or watering system are easily accessible -- even after the plants are in it.
Third, make sure the tank is well ventilated/ has excellent air circulation. If it is designed as an orchidarium than it should be ... but never hurts to make sure that it actually is.
Fourth, how easily obtainable are replacement parts? (For things like the lighting, air, and water systems if such exist.)
Fifth, you mentioned "for warm growers" ... so is some sort of heating system going to be necessary at part of the year?
Can't think of anything else at the moment that one should look into before buying one.