We took delivery of our boat (2019 R-29 CB LE) this summer and after thinking about your question, there is nothing I wish we had included but didn't and vice versa, but we are still in the process of making it our own. I do have a list of things we were considering during the build process that we opted not to do until we operated the boat for a while and some considerations that have nothing to do with the build process. Not sure if any of these apply or are useful to you, but here they are:
1. Considered an upgraded, thru-hull xducer: opted to see how the in-hull xducer worked first before cutting a hole in the hull. We did the factory delivery in WA (absolutely recommend it if you can do it) and the xducer worked great. Took final delivery from Pocket Yachts on Kent Island and sailed the boat home to Hampton, VA. The in hull worked fine in shallow water, deep water and throughout the entire speed range of the boat in Chesapeake depths. For now, I plan to go with the factory installed xducer but may add a temp sensor to a trim tab to get that piece of data. We don't fish, so this setup will be fine for us.
2. Considered a CB bimini: I know it's a must have for some and one owner on Tugnuts had a custom one made that covers a lot of area on the CB which is awesome. Based on the necessary height of the bimini, we wanted to get a feel for the best bridge setup before committing. Still in progress.
3. Considered a full boat cover: We keep our boat on a lift and I want to do what I can to protect the hull color between polishing/waxing. I got a quote from King Canvas and a local Hampton sailmaker for a full cover. The $4K didn't seem outrageous, but the thought of me trying to work a cover this size over the boat in a breeze made me realize I was the weak link in that idea chain. Instead, the sailmaker is fabricating a skirt that I can drop over the bow, strap to rails/cleats and secure at the water line with a drawstring. Still in progress.
4. Considered a second chartplotter at the lower helm/Garmin Fantom/Garmin Wx Antenna/AIS 600: I really gave these some thought...for redundancy, capability, safety and symmetry. Ultimately decided to go with a cellular iPad Pro 12.9 mounted with an easily detachable flush mount to the dash for now. We can run charting apps, check weather, access the internet, etc., all through sources independent from onboard sensors. Potential limitation would be cruising outside cell range, but think that risk is minimal for us, at least for the foreseeable future. The AIS 600 is the one that I'm still on the fence about.
5. Rudder indicator: Our AP pump does not have the required sensor port, so we would need the additional converter in addition to the sensor itself...about $400 or so. There are cheaper stand alone units that would require a separate gauge on the dash. We back into our lift, so I thought this would be a great piece of data. In using the boat, however, it's clear that rudder position doesn't matter when backing down in slow bursts, especially with the thrusters. So, I just center the rudder based on 2.5 turns from a hard stop in either direction and leave it there while backing down. I think I've finalized my decision not to add this.
6. Winterize or not: Part of the beauty of having a dual helm boat is being able to go out when it's cold without having to be outside. We are opting not to winterize the boat so we can take advantage of good days, but we did take some additional steps. First, we got 2 Caframo space heaters, one for the berth and one for the salon. I also got a Caframo engine room heater. I monitor the temp/humidity of each space with wireless sensors that I can read at the base unit in the house. So far, we've only had one really cold night/day, but it looks like the heaters are doing their jobs. I did put isolation valves in the transom shower water lines (under the access just inside the salon door) and blew the lines out with air. I also blocked the engine air intake passages with rolled up towels (and put a towel through the steering wheel to remind me to remove them). It takes a few minutes to de-rig the heaters and towels, but not a big deal. The space heaters are pretty amazing, so we are planning to use them underway or at the pier if the engine heat and reverse cycle units can't keep up, but we haven't fully tested them in the coldest conditions, yet. Not sure if they would be a suitable substitute for your potential Webasto upgrade, but worth considering.
7. Freshwater/Salt Away flushing: I have used Saltaway in the past and while its true effectiveness can be the subject of an entirely separate forum, I am a believer and have seen the results on the boat externals and engines. I also like the idea of using freshwater for the flush and rinse rather than salt water, so I bought extra seawater strainer caps and am in the process of modifying them with hose attachments so I can hook up the Saltaway mixer. Still in progress.
8. I may be looking in the wrong places, but technical information associated with basic D4-300 maintenance beyond what's in the owner's manual is scarce. Also, my usual go to of YouTube doesn't offer much here, either. I bought the D4-300 Service and Maintenance Manual and Workshop Manual (total of about $100 through Volvo Penta). These manuals have all of the information I will need to do the basic maintenance required and ensure I have the right torque specs, process steps, etc.
9. Considered a dinghy: Eventually, we will need one, but until then, we didn't want one blocking the stern view. Still considering the options here, but we may try an inflatable kayak for the near term and see how it goes.
There may be others, but these are the big ones that come to mind. Enjoy the process!
Mike