Your desires are bigger than your budget. Have you priced those two Roadtreks?
Traveling with a grandchild is always a romantic goal of newly retired grandparents. We went through that. It is actually more difficult than imagined because the design sacrifices you have to make that take away from 95% or more of your two person travels. Children have high curiosity at first and are good for a trip or two with grandparents and then it gets old for them. They just don't have that travel patience. "Are we there yet?" We had the seats but for sleeping it was the floor in emergencies (rain one time) or otherwise a tent.
The other couple at times concept is nice if you intend to use your B for other functions such as tailgating or everyday use. The most functional way to accomplish this in a Sprinter is an electric sofa/bed that has seat belts. The extra captain chairs Roadtrek and some others provide kill a lot of space for touring. Interestingly, some of the Promaster models accommodate more people, more functionally for the seldom done trips. I just don't think they are as adequate as the Sprinters for two people long term touring (more than a week). I imagine Promaster owners can give you another view about this.
A 24 foot Sprinter is as easy to drive as a 22 foot Sprinter. The difference is not noticeable. I've had both. Both are longer than the standard 20 parking stall so take some experience in knowing where to park. It doesn't keep us from ducking into a McDonalds or a Starbucks. We pretty much go everywhere including parallel parking on downtown streets. Obviously shorter is always better but shorter gives up a lot in my mind.
Most Bs have wet baths in that the bathroom becomes you shower stall. Again there is a big sacrifice for space otherwise. Some wet baths are more functional than others. As for a dry separate shower in the Sprinters only Winnebago Era and Coach House Arriva provide one.
Era | Floorplans | Winnebago RVs
Luxury Motorhomes - Fuel Efficient Downsized Class C (Class B-Plus) RVs - Arriva
Air conditioning? Yes loud. Despite what anyone says, if you are going to use air conditioning to sleep 8 hours you will need to seek a campground with a 30A shore power service. If you think an air conditioner is loud, a running Onan generator under your bed makes it doubly loud and vibrating.
Work desks are usually accomplished by turning around the driver's seat and having ample room behind for a desk be it fixed or drop down. Coach House Arriva and Airstream Grand Tours have good setups built in desks. Advanced RV has done desks in many custom configurations. Using a temporary table is another option just about everyone has.
Wifi or Internet connection is best accomplished cellular from a phone or device via creating a hotspot your computer can connect to. Wifi on the road is not dependable. I have an internal wifi network router with a booster to pick up any available wifi such as at a campground and then can connect all my devices (iPhone, iPads, laptops, etc.) to just the one connection just like at home. However, you have to have a reliable wifi source to pick up over the air. You can't improve their speed unless you want to connect at 3 AM in the morning when all the other users are asleep.
TV? I have I think the biggest TV provided for a B and that is a 24" model. Digital TV reception is usually good near metro areas. Only expensive private campgrounds like KOAs provide cable service.
Bed comfort? You just have to try them. Everyone is different in that regard. I like very firm. I have custom electric articulating beds I designed and Advanced RV developed and built. I'm on my third Class B so had a really good idea of what I wanted. My experience has been chronicled here: