Welcome to the forum. Well, I think you're getting the Enfield the right reasons! Simple, they are. There is more maintenance to do than any other new bike on the market but that's to be expected, of course.
As you mention regular maintenance and initial break in are important otherwise you may well suffer problems, either immediately or further down the road. I'm not the guy who can really answer the longevity questions you ask, as I haven't owned mine long enough to tell you. But with good maintenance and upkeep, You could expect at least 30,000 miles before any kind of serious work is needed, probably longer than that. I'm sure someone will chime in with a better answer. These bikes start to have problems when you try and increase horsepower or run them at high speeds. You have to always remember they're 50 year old technology, with a very low power output, and kept like that they're very reliable. If you do a minor job of jetting or adding a less restrictive exhaust, I'll think you probably help the bike some by allowing it to breathe more freely, but if you expect to rebuild the carbs, head and exhaust systems and produce twice the horsepower and try for 70 mph all day, then you'll be in for a nasty suprise and some expensive fixes.
Think slow and relaxed, plodding down the road, admiring the scenery, and the bike will do you just fine.
I plan on messing with the carb jetting and exhaust in the next few months, along with removal of most of the emissions garb that restrict the bike. From what I understand, its fairly straightforward. If you can set your yamaha up, I'm sure you'll have no problem with an Enfield.
There was a discussion earlier about how and when to do some of these modifications. Many say wait a while until the break in period is over. The bike will change considerably as it breaks in and you'll have plenty to do maintenance wise as it is, especially if you follow the Pete Snidal procedure. Also, if you change of bunch of stuff from stock and you have to take the bike back for warranty work for any reason, you may problems. Just a thought. I would think if you wait until you have around 2000 miles on it, it'll be broken in enough where its not going to change much and you'll have less screwing around with dialing stuff in.
It took me almost a full season to get to 2000 miles, as the initial break in has you riding at 30 mph for extended times. I highly recommend you do this, but at 30 mph, miles don't pile up quickly at all! Without hours and hours of spare time, it took me a while.
Have fun and good luck