Regarding Montebank:
I'll usually go with 2 as fast as I can. I'm not sure if there's any discussion on going to 3 or not. I suspect on some boards with no trashing it could be a good idea to get 3 montebanks to ensure he is hit harder than you (and also since he'll be less likely to discard a curse with no trashing, and you'll be less likely to collide with no trashing available). I'd say 2 monte's on trashing boards and 3 on non-trashing boards, but I could be wrong.
More specifically, when is it too late to buy one? The later the game goes Four things are going against the purchase of a new montebank. First, your deck is bigger so you'll draw it less frequently. Secondly, you'll draw it fewer times to use it because the game is ending soon. Thirdly, when you use it, the impact is smaller, since his deck is likely bigger now. Lastly, your deck doesn't want "Silver" anymore, you need a higher money-density card to access provinces more reliably.
Also, consider that the timing of turns 7, 8, 9, 10 will depend on the timing of when the shuffle happens. For example, turn 7 = turn 10 if the shuffle happens on turn 10. But turn 11 = turn 15 if then the next shuffle doesn't happen until 15. So a buy on turn 7 is the same as a buy on 10, and similarly, a buy on 11 is the same as a buy on 15 (in this case).
I would gut-feel that anytime after the shuffle that happens around turn 8-9 or so is too late for a montebank. If you can squeeze one in say on turn 8 before the shuffle, go for it. If your decision to buy it is just AFTER the shuffle on turn 8 on a non-trashing board (i.e. kinda big deck), then don't buy it. For example, buying it just after the shuffle on turn 8 and your deck is slightly bloated, this could mean that you won't even shuffle it until turn 12, then not even draw it until turn 15! So even while both cases discuss a turn 8 purchase, the timing of the shuffle has an enormous impact on your decision to buy it or not.
Again, this is completely game dependant, and likely depends on many things such as: Your deck's ability to cycle itself, overall trashing, Province/Colony game, Overall speed of the kingdom, his deck status in the midgame, current status of VP (i.e. earlygame, midgame, endgame) etc...
Regarding creating Villiage Engines in Province games:
Since it takes time to set up villiage chains (i.e. + actions/strong terminals), the only way you are going to catch up once you get your engine running is either:
a) your engine provides you with a VERY consistent turn 12-16 in which you can almost guarantee to buy a province in each, or
b) your engine will allow you to buy multiple cards per turn. I.e. such that you can overcome a 0-2 province lead by buying multiple in your last few turns.
I believe this is what you should be looking at when deciding to go villiage-type engine (or really, any action based engine) versus simply big money plus a few strong terminals.
For an example of case a) Minion + trashing engine (say such as Loan + Minion). It's a strong engine that once you get it going, it will likely buy a province 4-5 turns in a row. This type of engine can surmount the 0-2 (ish) province deficit by being more reliable on turns 12-16 (or so). I.e. big money may go 2-0 on you, but will stumble trying to get it's 4th province, whereas you'll just smooth-sail to 4 provinces easy. The key to a single-buy engine such as this is to accurately pick when to buy your first province. If you wait too long, you'll be stuck flirting with the PPR (Penultimate Province Rule), and maybe having to buy a duchy with $8.
For an example of case b) Grand Market/Vault. (City+X is a more obvious example of this type of engine, but I don't want to derail this thread as much, and GM/Vault is a similar feeling engine). This engine will take a little bit to set up, but once you get it going, it will buy 2 provinces on each of it's final turns (likely). you'll likely start 0-2 provinces to big money, but win something like 6-4. This first big turn should happen somewhere around turn 11, 12, 13 or so, and you'll likely follow that up with double-province buys in the subsequent turns (or at least province + duchy)
These are just a few examples of things you should look for in a province game to assess if you have time to set your engine up and come back from behind a lead that something like big money + terminal will gain on you.
For an example of what (probably) would NOT work... something like Villiage/Smithy/Bank on a board with no +buys. Sure you can draw a ton of cards, but what does it matter if you end up hitting 10-15 gold in a province game with no additional buys? On this board, the simple Smith + Big Money player will easily beat you.