I can vouch for the 9 speed box - got one on my c class and it is amazing. That’s said, our van is the 220 and it drives fine for our needs. We don’t have it as a daily driver but use it for trips and hols only. It coped with trips to Spain and France back loaded up with us and the kids no problem.
Re microwave, I guess the challenge would be where would you put it whilst still having access to the other units whilst allowing the microwave to be usable? You could try and find a custom conversion company - buy a VW camper mag and you’ll find loads of companies in there. Or Might be worth looking at other options on the market, eg going to Westfalia, harbour creek iirc are a dealer. Camping and caravan show in Oct another option.
Not sure how long you be staying in the van re need for a microwave but if considered an awning maybe a small portable microwave on the table in the awning? Another option might be to try a ridge monkey pan which basically is a pan with a lid and seems to cook fast and cuts smoke + fat spits down massively.
heater. worth checking out the purpose of and how long the battery coolant heater works to ensure it meets your needs - ie is it capable of warming up the cabin for a short period or suitable for many hours. And if you are not connected to electricity how long it would last.
If you can get a really good deal on a 250d with heater and options you want I can’t image you’d be disappointed with it.
Do post pics of what you guys end up getting!