If the used rims are still laced, I would just use the old wheels.
If you insist on using old rims, then at least lay them flat on a glass table to see how straight they are. Just cutting out the spokes on a wheel to get at the rim can bend the rim beyond repair.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1