Lots of good information here. I can add from my experience with owning an 18 ft boat with a 150 on it. It was fiberglass but I don't see that making a difference here. I fished walleyes trolling with the boat mostly and it had a 24v terrova and it would do fine for a full day trolling at 1 to 1.5 mph with the wind. The batteries are heavy and take up allot of room. IMO a 36 volt system would be overkill not to mention another charging bank needed. Lots of additional cost and weight with 36v system.
I did not have a kicker on that boat and never felt like I needed one. Again more weight and cost if you don't really need it. My current boat has both the 36 volt and a kicker and it needs it. My 18 foot just didn't require it.
I have done both in regards to rigging and you will likely do a better job than the dealer would.
Also I strongly encourage you to do whatever it takes to get in the boat model you are interested in as well as a second pick. I drove a couple of hours each way twice to do this and it changed my mind on the last purchase. Best thing I could have done.