avantgarde literally means "bare bones" - its one rung above the taxi spec - the nicer versions are usually dubbed elegance and can have a laundry list of options or just one or two.
From experience, dont buy the leather seats option as they are not all that great and a bit hard to keep up with. Also do not buy the AMG wheels package as your alignment is not setup for it and it also usually wears out the expensive tires within 30,000 kms no matter how easy you drive.
The most useful advice here would be to be on a 4 on a scale of 1-10 of your mechanical work aptitude. This does not mean you are excellent in naming parts but can actually understand how the parts install, work and perform, this way you can measure the work of a technician, workshop etc. This ensures a good long service life. Another thing is that I would suggest to read a mercedes benz service manual and try to understand what they mention. The only problem is that the writers consider the technician to be very well versed and already educated in working on cars - e.g. while repairing the suspension work, they expect the technican to already know that torquing down means - hub in rest position - they dont mention it as it is elementary knowledge.
similarly they assume that the user is smart enough to know that a machine will perform best when in "normal condition" - this automatically eliminates the kind of users that come and complain that their cars bump shift when starting from dead cold, this is not a fault according to MB or any manufacturer infact. Buyers of MB expect some out of this world experience or spaceship travel when they see a three pointed star car.
My reason of owning this brand is that it will not wear you down in long journeys, it can be serviced back to 100% spec and its service intervals are at minimum 3 times longer than other brands.
I have about 190,000 miles on my car (304,000 kms) - its a W210 - E320 model year 2000 (first facelift year) - it runs sweet and has no worries at speed or in long journeys across the desert or in snow.