Much of this depends on what you purchase, it's age/condition and what you plan to do with it. New units come with warrantees and after this period you should expect a long life of service before items begin to need replacement.
Vintage units are typically cost effective to purchase but will require some level of "investment" to bring them back to working (and safe) condition. Once restored they will again give you a long stretch without much in the way or large scale maintenance. If you can do the work yourself, it can be very cost effective. Even outsourcing larger jobs can still give you a very nice trailer for less than new, but if you don't know which is the business end of a screwdriver and end up taking it into the shop for every little thing you will feel about the same as you did with your BMW. If this is the case I would encourage you to consider a new or newer AS rather than a well used AS.
You didn't specify trailer or motorhome, but obviously anything with an engine and drivetrain will require a higher level of maintenance and a corresponding budget for repairs.
Again, what you do with it will determine your maintenance needs. Full timing in one, traveling every few weeks will place additional maintenance burdens on your internal systems and your suspensions, shocks, tires etc. Weekending in one will barely use anything in the coach.
Having said all of this, most of us believe in investing in these beauties because they can be restored and re-used over and over. Most other brands would have fallen apart long before the last restoration of any vintage AS.
The unique contruction of the shell itself will, without damage and with proper care outlasts just about any other brand on the market. After all, AS will tell you that's why most of the RV's they've built over the years are will on the road and in use today. Not bad for the oldest RV manufacturer in the country.
For us, we puchased a mid eighties motorhome in need of TLC. We bought it right, but invested a lot into it over the first two or three years. Since then our maintenance curve dropped considerably and now, rather than spending money on parts, systems and safety we're making the modifications and upgrades we want to make to it. At some point this process will be done and despite budgeting for major wearable items like an engine, tires, tranny, etc. we'll own a luxury RV motorhome with no payments that turns head wherever it goes. Even now it doesn't eat parts or require gobs of cash to keep on the road. (if you take fuel cost out of it
Despite everything we've invested, we're still well below what a newer, less exciting motorhome would have cost from another manufacturer.