I'm north of that building my 650b Terraferma. But how is this (or the $300 to build 650b wheels) different than building a high-end conventional road bike on a custom steel frame with custom spoked wheels? The rim costs are not different. Synergy rims cost about the same (in context of a $4k bike) in 700c and in 650b.

If you're going to built with Hetres and fenders, you need a frame that is wide enough between the chainstays near the chainstay bridge to clear the tire and the fender. We can argue about the right number and the right fender, but I think you need at least 6 mm on each side of the tire (42+6+6=54), and a total clearance between that and 60 mm to handle fenders not larger than 60 mm Grand Bois. Similar width is needed near the top of the fork blades.

That requirement alone makes a 42 mm fendered bike hard to make without a specialized frame. Vintage Treks don't have nearly that much width in either position. Between the chainstays both my 1984 610 and my 1983 620 measure about 47 mm. So there's an available 2.5 mm clearance for the inflated tire alone. Whether or not its adequate depends on correct rear-triangle alignment, wheel trueness, wheel dish, accurate tire mounting, and accurate wheel positioning based on the dropout adjusting screws. Break a spoke and you might not be riding home. Needless to say, adding fenders in this case is a non-starter.

I'm interested in converting one of these to 650b, but I think the opportunity is to go for 650x32b with fenders, on a vintage trek. But this only speaks to lateral wheel and fender clearance, not to trail adjustment or brake choice. I'd probably go for Mafac CP pivots front and rear and install Mafac Racer brake arms. Racers work on 27" tires that are 32 mm (1 1/4") wide with fenders, so they should work on 650x32c.

Why would I do this, why not just use 700x32c with Honjos? Well, I've already tried it with 45 mm plastic (Zefal) fenders, and that bike just does not have enough radial clearance for that tire/fender combination. It does work with 28s, which I use now.

So for a vintage Trek, it's an expensive way to go to 32 mm tire width with good fenders, and more if you're going to adjust the trail to either make it like Trek had it for 27's or 700c's, or you're going to adjust it for a front load. But these fork issues are significant no matter what frame you are going to do it to.

So I'd say if you really have to have fenders and 32 mm cushiness, one of these Treks is a good choice. I don't think it can get you "home" if you want 42 mm with fenders.

I'm assuming no frame work (replacing/recontouring chain stays) beyond adding brake pivots and possibly rear cable housing hangers to convert to centerpulls or cantilever brakes. If you're going to remake major sections of a bike frame, anything is possible, but it just gets more expensive.