Since your frame is already cold set to 126mm, go ahead and use a 6 or 7 speed freewheel. There's really no reason not to use a 7 speed.

A standard NDS axle spacer is 5mm. Your LBS may give you one for free.

I would use 14/15 DB spokes. They are lighter and yes, stronger than straight 14 gauge spokes.

Older tubular rims work fine. If you're worried about your weight (I'm 200#) find something with a semi-aero profile. I ride old Super Champion box rims on the TdF, but have one bike with Araya Aero4 and one with Mavic Mach2 CD2. Both of those are semi-aero and presumably stronger. My guess is the CD2 would be easier and cheaper to find.

I run Vittoria Corsa EVO CX on 2 bikes and they are wonderful tires, but they do wear faster than the Hutchison Reflexes I have on the other. I've managed to buy them at reasonable prices off ebay.

I use glue. Haven't tried the tape.