A good 91/30 can be had for about $140 at least in my market (North Texas). This should include the accessories such as cruciform bayonet, weird oil can and cleaning pouch and tools. Most shops I've gone to keep one on the floor and you get the luck of the draw from the cases in the back. I suppose if you have a good relationship with the shop owner, maybe you could convince him to select one with matching serials but I've never tried this.
Ideally, matching numbers are best. There are four spots: Buttplate, magazine floor plate, bolt handle (under cocking knob) and receiver. There will also be a serial on the bayo but because of the way the russians arsenaled these, its rare for that to match as well.
Other than that, Tula arsenal is preferred to Izshevsk and hex receivers (pre-WWII) are preferred over the later ones. There are resources online to help you identify the proof marks to tell which arsenal produced the rifle. My first one is a 1942 Izshevsk and it shoots just fine! Also got pretty lucky on a heavily figured birch stock. I've attached a little pic.
Anyway, about cosmoline. Yes its messy and it'll most likely be all over everything. I completely disassembled the MN and degreased the metal. Its pretty easy to take down and there aren't a whole lot of parts to it!
As for the woodwork, the stuff is going to be impregnated in there so you'll be dealing with it for a while. What I do is wipe off as much as I can from the wood, wrap it in a towel and leave it in my car for a few weeks. The hot Texas sun will do quite a bit of the work for ya, but it'll make your car smell a bit. I don't mind this but you may. After a few weeks of that, you can pretty much get the rest out if you are planning to redo the stock. I had to get the ugly shellac off mine and I went to a simple BHO finish along with a beeswax stock wax.
Ammo is still plenty available. You can get a 440 round spam can from CTD for I think around 90.00. I pick these up now and then but I shoot privi partisan because its a little cleaner and I like to keep the spam cans stored for later.
The first thought that went through my head when I fired the MN for the first time was "why don't I have ten of these?"
Enjoy