I think whether you want to do upgrades or not is dependent on a couple of things, but mainly, what your expectations are for the pistol, what you'd like to do to improve certain performance characteristics, and finally, what your budget is.
In this same Upgrades & Modifications forum, stickied to the top is a thread I've created that deals a lot with these different performance and aesthetic characteristics, as well as covers almost all general questions you may have with this particular series of Marui GBB.
http://www.airsoftcanada.com/showthread.php?t=94413
Post #3 lists out all the different performance characteristics that I could think of, but I haven't gotten around to filling out the descriptions.
I've noticed a discussion of metal upgrades, so I'll comment a bit on that: It's true, a good quality kit can cost anywhere from $240 to almost $700, depending on the make and model. Also, depending on the kit, you may be required to purchase additional components to support such an upgrade. If you're on a tight budget, either wait it out, or do what I wouldn't normally suggest... which is to just buy a WE model, which is already full metal to begin with.
If you're on a budget but want to upgrade a Marui MEU, I recommend the VFC metal kit. The markings aren't accurate to a late model MEU SOC, but for the price and for the build quality, they're really hard to beat. They were also the easiest kit to install out of all I've tried.
On the other side, if you want a premium quality kit with superb construction, fitment and finish, check out Nova kits. You'll pay through the nose for them, but they're king of the castle. Expect some fitment issues as you install with Nova kits, though.
I would stay away from Guarder kits... the fitment is okay, but a little bit of work is provided. What irks me about them, is that for the price of a VFC kit (or a little bit over, depending on your source), the fit and finish is not as good, the quality isn't as good, and what bugs me the most is that a destructive modification is required to be made to the blowback unit. This modification renders the blowback unit useless with any other brand of slide than Guarder. Poor engineering, imho. Right out of the package, Guarder rails require filing to some areas JUST to get the slide to mate with the frame. This shows poor manufacturing. To top it off, once fully assembled, there is a bit of play. However, if you're willing to go through the fitment process, Guarder kits can provide a very cost effective solution.
If durability is an issue, I'd say you're worrying over little. The TM plastic is very durable.. I've yet to see a Marui single stack plastic slide/frame break under normal use. If anything, the first bit of wear you'll experience is to the slide catch notch. However, if you learn/train to use the "slingshot" method to release your slide rather than to hit the slide catch button, you'll greatly increase the durability of that notch.
Beyond that, metal is mostly costmetic. It adds a nice weight, it gives that cold metal feel to the touch. You get really nice *clink*clank* sounds as the slide moves and rocks the chamber back and forth. These qualities are worth the extra cost to many users, myself included.
I also noticed a mention of weight and balance in this thread, so here's my comment on that: From my personal experience with Marui 1911's, is that they feel too light, and are very grip-weighted. A real 1911, is much heavier, and the weight is balanced over the trigger more than it is towards the grip (even with a fully loaded magazine.) For myself, I go the full metal route, and try to put as much steel in the muzzle as I can to bring the weight forward. In one of my particular builds, on top of a steel barrel bushing and steel recoil rod bushing, I've also opted to go with a bull barrel and full length steel guide rod to bring that much more weight forward. It's still grip heavy, but the balance is closer to the real thing.
In the end, try the pistol out first... see what you like about it. See what you don't like and then you can look in to options to fix or improve the things you don't like... or make the things you like about it even better.