Congrats on the early delivery!!
It seems common during the past couple of months that people are getting early MINI deliveries. Good news for all!
Regarding the wool mitts, the most important aspect is to make sure it is 100% wool. There are several synthetic wool mitts out there- these will surely scratch so you want to avoid synthetic wool for sure! It must say "100% wool" or it probably isn't.
There are different grades of sheepskin/wool, but I can't say definitively to avoid lesser "brands" though. As long as it's 100% wool and it feels good, it's probably ok. Hard to make any guarantees here of course, but I think you're probably fine if it feels ok. Just feel a bunch of them- they will vary a bit no matter what brand you're looking at just because they're natural. I would aim for the softest with the longest wool fibers.
Regarding the microfiber sponge, that's a tough call. Microfiber is really best on clean or fairly clean surfaces. It's widely debated whether or not it is the best option for washing a dirty surface, even with alot of soap. Industry wide, the jury is still out on even microfiber mitts- although I think the longer, looped style is probably the best of the MF variations.
If you're itching to wash the new MINI once it stops raining, I would try to find the best 100% wool mitt you can and use it with a high concentration of car wash soap. The high concentration will give you a little more lubricity and will give you a little more "wiggle room" to compensate for the possible lesser mitt. Bottom line: I think you will be just fine using this combo. I'm sure it's just too exciting to wait!
Last, buy a second mitt (wool, cotton or MF) for the very bottom of your lower panels and wheels. Then, never mixup the 2 mitts! Even if the "wheel mitt" looks clean, you don't ever want to use it on your paint (except for that very bottom part of your lower panels, IMO).
Congrats again and have fun today!!
-Heather
