Hi Iain!
At the fork blades, I will allow 5-8mm minimum clearance each side between the widest part of the tire and the inside of the fork blade. Because I am risk averse and don't want the front tire to foul the fork blades, I tend to go with 7-8mm minimum on my own 700C road bikes.
At the rear, you've got a bit more leeway; most 700C road bikes have either ovalized chainstays or the stays are indented for greater tire clearance on the inside faces opposite the tire. There, I have had no problem with 4-5mm minimum clearance on each side.
These measurements are for tires that caliper to a true 32mm to as much as 35mm at their widest point, and are my measure of the gap between tire and tubing at the narrowest point. Each of my bikes have indented chainstays which helps with tire clearance.
The main thing is you want to allow for the unexpected -- mud buildup, a bunch of sticky wet leaves, or a wheel being knocked badly out of true by hitting something unexpected. If that happened at the rear, at most the tire would scrub the inside of the chainstays and you'd lose a bit of paint or the wheel would possibly lock. Between the fork blades, things could become serious quickly if the tire/wheel bound up (faceplant). Again, because I am risk averse I won't go closer than 5-8mm clearance at each side up front, but I have seen others do fine with less -- on the order of 3-4mm per side -- but I wouldn't want to risk it myself.
As for spokes, I've had wonderful luck with unbutted 1.8mm DT spokes and Wheelsmith nipples in my narrow box-section rims (DT nipples tend to discolor over time in my own use), but again, I'm really easy on wheels and equipment. I built up my heavy-duty touring wheels using wider 700C Alex Adventurer rims for the Miyata with 36 unbutted 2.0mm Wheelsmith spokes and nipples laced cross-3 on Bontrager sealed-bearing 9-sp hubs and they held up wonderfully both on- and off-road during my 2010 Great Basin desert tour, where the bike had a loaded weight of 49.4kg plus my own 78kg for an overall total of 127.4kg. I used 32mm Bontrager Select K road slicks and had no problems except when I broke the Kevlar belt in the rear tire by running over the leg of a portable highway caution sign when forced off the shoulder by a car. The wheels ended the trip true as could be -- to my usual standard of 0.1mm radial and axial runout.
For your intended use and weight (keeping in mind your eventual goal to add more weight with a touring load), I would suggest either unbutted 2.0mm spokes or butted 2.0/1.8/2.0. Again, I'd go with 36 spokes laced cross-3. Keep in mind this is for your less heavily stressed front wheel only rather than a pair.
As for brands of spokes, I've built with all the big names and have found the quality pretty much identical between stainless DT, Wheelsmith, Asahi, and Sapim. I think Wheelsmith are the most thoughtfully engineered details and have the most corrosion-resistant nipples that are also the most resistant to rounding under high tension. Sapim's polyax nipples are self-centering to a degree and are a requirement when you are running low-cross spoke lacings on high flange hubs (i.e. cross-2 spokes with a Rohloff hub and Rigida Andra rim, which also has socket nipple seats in the inner spoke wells), but the nipples are a bit soft and don't seem to handle very high spoke tension as well as Wheelsmith nipples in my experience. I think it is a matter of matching features to need and one brand of spokes/nipples is not clearly better than another except in specific applications. Again, if the wheels are properly built with even high tension and protected/cushioned by tires appropriate for need, they should hold up fine for a larger rider who uses them with reasonable care to avoid hitting potholes and such square-on.
As for tires -- keeping in mind the clearances mentioned above -- I'd go with 28mm measured (actual) section width for go-fast use unladen on good roads (not that they'll necessarily have lower rolling resistance, but you'll notice the difference in acceleration due to reduced weight), with 32mm for general use and 35mm for touring; 38mm for heavier loads on bad roads if they'll fit.
You haven't mentioned the kind of bike you'll be outfitting, but if it has ramped dropouts at the rear, you can gain a bit of clearance by moving the axle rearward just a bit, though this may affect shifting precision to a degree. Remember, some tires are wider in the upper third of their profile, while others have side lugs or a tread cap you'll need to watch for, so clearances will vary by the individual tire chosen. A slick will give the greatest interior air volume for a given size 'cos you won't have to allow for tread lugs. If you go with wider tires, keep in mind you'll probably have to deflate them to pass by the brakes when installing or removing a wheel.
Each custom wheelbuilder will weigh a number of variables in recommending and selecting components and build design. When I build wheels I like to know something about a rider's needs and style as well as weight and intended use. That way, I can fine-tune things, using everything from differential lacing to mixed spoke diameters and crosses and choice of components to meet the need. I'm sure your new wheel will be fine; Thorn have built a lot of wheels for a broad range of customers and should have a good sense of what will work well for you.
On a personal note, I want to offer every encouragement in your journey back to full cycling fitness. That was surely a big "off" you had, and I'm glad you weren't severely hurt, though it must have been tough to lose the bike and I have a feeling your injuries were no picnic. Nice to think you'll be back in the saddle again soon!
You're welcome to PM me if you have any questions or want to toss around ideas.
All the best,
Dan.
