Sorry can't give an exact answer but you might look at the 17 vs 17+ comparison of versions chart (link near bottom under 'Extra information') from here: http://www.magix.com/us/movie-edit-pro/plus/ . Features that were only 'plus' tend to get added to the regular version at the next upgrade. If MPEG-4 requires activation in 17 it almost certainly would in 16. I'd guess every regular user says always go for the plus version.
Version 16 might be available cheper now but the changes in performance between 16 and 17 are supposed to be significant. Try the free demo download.
There used to be a 16 vs 16+ page but that gone now that 17 is out. Not sure about 14 & Win7; it came out before 7 IIRC. The other forum had some discussion but it's hard to tell what is an OS problem rather than a machine / codec specific one. MPE 17 is definitly ok for Win 7 and is supposed to make better use of the extended RAM.
I'm also unlikely to use the multi-cam feature, but HD will be the new standard so why not be prepared for it? If 17+ has 5.1 surround sound and regular 17 doesn't then for sure it won't be in regular 16 (not even sure about 16+). I think surround sound is great to play with.
I know price has to be considered but I'd certainly have a look at some of the deals available. A group called downloadcrew are offereing the download version of MEP17+ HD for ~$45. (Never heard of them before searching!) Many say always get the box set but that is often unavailable here 'down under'. I just use up bandwidth downloading all the extras (more effects, templates eetc) as I need them.
Well, the difference between the 17 and 17 Plus can be downloaded as a pdf from this link. You asked about the 16 differences but I would tip on AVCHD handling, the inclusion of MPEG4 codec, whereas you need to pay in the 16 classic version.
If you are going to upgrade, go to 17, perferably Plus, because what you may now feel is not relevant will change quickly in due course. AVCHD handling is far better in 17, especially Plus version. 16, in comparison, is a generation behind 17, which is not what you want from a new product.