The best spacers are dead cogs out of old cassettes, because they are free.
I went to a shop to do that to my new rear wheel last week, and they had used up all their cogs making decorations, so we used some bottom bracket spacers. Seems to work ok too, but without the Heath-Robinson element I liked.

I don't know about the wheel-changey thing- remember 8 speeds onto an old 7 speed hub won't work too well unless you can hash it.

Also, the Tiagra wheels are supercheap, but have you considered hand mades from Ribble, or Dave Hinde (both don't have great customer service reputations), or Chain Reaction cycles? A Sora hub with a Mavic MA3 rim would be cheap and stronger to boot! Also, M:Wheels do a pre-built version of the same; my experience of a similar wheel was great (until it was knicked)
Dave