I am not a lawyer, you should consult one before making any business decisions.
Anyway, if I did so, I would want them to (obviously) look like the creature they represent, and be armed and armored as per the MM description and art.
Using the names orcs, kobolds, etc is easy. All you have to do is include a copy of the Open Gaming License with each product. You also need to get in the habit of saying you are referring to the SRD (and actually look at IT when you're creating stuff).
Once you decide you're going to arm and armor them with the info depicted in the SRD though you are right back to using the OGL. The d20 logo is out, since the guide specifically prohibits miniatures as I recall.
Of course, I've seen illustrations of various MM creatures in various third party products, but I was wondering if the laws change if the image (or model, in this case) is the sole product being sold.
Yes, the beholder is one of the creatures that has permeated itself almost into mainstream culture. This is iffy. Basically, everything you've seen is a derivative of the original so everything is a violation. For it to be otherwise you'd have to prove you didnt look at their stuff when you created yours. Kind of hard to do if you play the game, isnt it?
This is one of the reasons we strayed away from a lot of the pieces in the SRD that were stripped from the Monster Manual. Without the descriptions, there simply wasnt enough to make it look like we all knew it should look like, without being a derivative.
All that said, are you ever going to be able to crank out enough minatures to make it worth your while to start a business? Are you going to be able to sell them? Would you be better off trying to sell your finished pieces to someone like reaper or another mini manufacturer. Starting and running a business is a lot of work. If I had it to do over again, I probably wouldnt have started DSC. At the very least I would have approached another company (like MEG) about doing an imprint. Not because I dont love what Im doing, but because the administrative overhead is a real drag. Taxes, taxes, and did I mention taxes? Paperwork and reconciliation, the whole nine yards. Think hard before you jump in. And good luck in whatever you do.