The best i can say here is that i share your concerns.
First of all, you are correct that whatever brand of PC you order you will get pre-installed software. Also in the case of Acer this will be true. It is your choice however to decide if removing this unwanted software is faster and more easy then a complete re-install. My first guess would be removing the software, but thats my personal opinion of course.
Second: are you sure you will get an (oem) installation kit with win 7? I'm afraid Acer will give you certain driver cd's and additional software, and the pre-loaded win OS will be on a (hidden) partition.
Third i think it will not be possible to use a different win7 os version CD with an OEM installation key. I'm not sure about this, but it has been allready the case with XP that your serial was not accepted unless the version (SP pack, Home, Pro) was the same as version of your installation key. (in other words: a Home installation will not work with a Proffesional installation key)
If this is the case also with OEM or open license (and i'm afraid it is), then you are not able to install unless you use another (clean) OEM version of Win7.
if however the installation is succesfull, i don't expect the actual activation will be a problem since that mainly involves checking hardware components.
Please keep us informed since i'm interested in the outcome.
regards, Mast