Okay, tlarkin, let's have a shot (sorry, couldn't resist) at answering your questions.
The pros? They are tough, inexpensive, hard-to-break firearms (since they exist as original length rifles [the M1891], Dragoon-model rifles, shorter-barrelled rifles [the Model 91/30], and carbines [the M38, M44 and 91/59] ) made in about a dozen countries. No matter who made them, they are reliable. There is plenty of military surplus ammunition available for them, plus modern production soft-point rounds for hunting. They are relaible under any conditions; a standing joke about them is that when you get yours covered in Cosmoline, it was probably last cleaned in Berlin in 1945. But think about it: where else can you get a rifle capable of taking any game animal in North America except - maybe - a grizzly bear for under $100?
There are aftermarket add-ons available, and most of them fit any Mosin. Most add-ons don't require a gunsmith; you can do it yourself. The only exception to this is bending the bolt if you plan to install a short eye relief scope or a sniper scope, as opposed to the more often seen long eye relief scout scope setup. Parts from one Mosin will fit any other Mosin, no matter which country or arsenal made it. This also is a big plus.
All of the milsurps are chambered in 7.62x54R. There is lots of surplus ammo out there to buy. For instance, I just paid $51 for 300 rounds of Bulgarian heavy ball (180 grain bullet), delivered. Try and match that price and quantity for, say 7.62 NATO, .30-06, .303 British or 8mm Mauser! it means you can shoot more, for less. There are Mosins that have been customized for other calibers; but they are usually one-offs and are very much the exception to the 7.62x54R rule.
What should you look for? That depends on how well you can calculate what's under the Cosmoline if it's newly imported. Look to see if the lands look sharp. Feel the bolt face and extractor, feeling for burrs. Open and close the bolt and see how stiff it is. A stiff bolt isn't necessarily a deal-breaker for a Mosin in Cosmoline, but it does mean you will need to detail-strip it and thoroughly clean it of Cosmoline. See my sticky in the Mosin forum on cleaning milsurp firearms. Look to see if it has been counterbored. Again that's not a deal-breaker, but I feel counterbored guns aren't as valuable as ones that have good, solid rifling all the way to the muzzle and the price should reflect that.
Look at the stamping on the receiver. There will be a bunch of it, but what you want to see is the year it was made. If you have a choice, try to get one that is from the period between the two world wars. In peacetime, gunmakers can take a little more care that they do in wartime, when the emphasis is on producing as many as possible as quickly as possible. It shows in the finishing. I've seen wartime Mosins that looked like they were put together out of rough castings, the machining was so coarse. It doesn't mean they won't shoot, but they sure aren't as pretty! Look also at the wood, and see if there are any repairs to it. If there are, that should lower the price though not the shootability.
Assume you are going to have to comeletely clean the rifle. I've seen very few Mosins even at gun shows that weren't still in the Cosmoline. Again, see that sticky.
What whould you be prepared to pay? Well, I just bought two from Southern Ohio Gun for $200, including shipping and the hand-pick fees. The going rate the local gun shops want for a 91/30 hereabouts is around $130 plus tax out the door. You can do better online, but remember to add shipping into the total. For example, Classic Arms is selling round-receiver 91/30s for about $80. Even if shipping is $20, you still come out ahead. Of course you can't examine the rifle first when you buy from an online dealer, but most have a 3 day return policy if you are really dissatisfied. Talk to them on the phone before you put down your money and find out what their policies and terms are.
I also note most online dealers will throw in an ammo pouch, military issue cleaning kit, sling and bayonet on the deal. Plus which, they don't charge sales tax unless you live in the state they do business in - another savings for you. Gun show dealers may or may not give you those extras and will charge sales tax.
I hope this answers your questions. If it doesn't, put in some time reading the various threads on the Mosin forum. Most any question you can ask has been dealt with on that thread before.
I look forward to welcoming you to the ranks of Mosin owners, tlarkin. They're really fun guns! And there's something special about shooting a piece of history, too. That's the attraction for me.