Adam-
If you're trying to save as much as possible of the 800 pound maximum weight for other uses, make a chart of all the brands of engines that offer 35 hp models and study all their characteristics.
Manufacturers can't make a separate engine block for each individual hp model, so you will find (when you compare their 35 hp model with that same brand's 25, 40 and/or 50 hp models), the the 35 may be the same basic block as the smaller 25, or the larger motors, and have the same weight. For this reason, 35 hp motors from different manufacturers may have considerably different weights. If weight is the single over-riding consideration, get the lightest 35 you can find that you find otherwise acceptable.
Note: The E-Tec motors from Bombardier are direct-injected 2-strokes, and are generally considerably lighter than 4-strokes, as much as 100 pounds in the larger75/90/115 hp motors. This weight saving comes from their lack of a valve-train apparatus. You may want to consider this alternative, as the weight advantage is certainly with E-Tec.
Additionally, make a chart out of what you plan to carry in the boat and actually weigh out the items whenever possible. See how much of the 800 lb capacity is left for your engine.
From what I've read around here for the past 15 years or so, all of the new 4-stroke and direct injection 2 strokes seem reliable and generally without major problems. It would be hard to make a bad choice, if this is true.
I'm sue more advice will be forthcoming.
Good Luck!
Joe. :teeth :thup