Mirrors on the front side of the dojo, so that your students can have an "imaginary" target to punch, kick, etc. Also, they can see their own positions.
The floor type really doesn't matter, unless you are doing throws and takedowns. A universal (but expensive) solution is to cover the floor with the "jigsaw puzzle mats, since the surface will be firm enough for regular techinques, but at the same time, a good bit more forgiving for throws and ground work. One bit of warning: the jigsaw mats can be a bit slick at first. Just let your students do the work, and wear away the initial layer of silicone protectant, and after a month, you'll have a great surface.
Alternatively, you can train on a hard surface normally (be it hardwood, concrete, carpeted concrete, etc), and then break out a couple of long sets of folding gym mats when it's time to do ground work / grappling / throws. This way is significantly cheaper than the jigsaw mat method, although not quite as convenient. Still, with several students, you can have them lay them down and pick them up in a very short time.
Other equipment you could really use:
Kicking and punching bags.
First aid kits
Refrigerator (you can store and sell bottles of Gatorade from there at a nice profit, and there's nothing like that big blast of icy cold drink after a hard workout)
Food and drink items: Gatorade and bottled water sell quite easily. Work your students hard, so they sweat, and they'll buy from you.

Also, having access to some quick snacks can save you a lot of $$$ during the day. It's cheaper to gorge on 75 cents worth of potato chips, than it is to get a 5 dollar Chinese takeout meal (although not as satisfying), when you're calorie starved. Believe me, it will happen...
Filing cabinets: All of the paperwork you want to maintain on your students has to go somewhere. Also, tax-related documents need to go somewhere safe.
A lockbox: Yes, martial artists are generally a good wholesome lot of folks, but once in a while, there may be a bad apple that might think of trying to steal the smaller, more valuable items.
A camping cot: Don't laugh. There will be times that you will be at the dojo, perhaps during a slow day, and you need that quick nap. Also good for cases where you might be stuck at the dojo.
Extra toothbrush, and various toiletries, along with a change of clothes. Just in case you're stuck for a night.