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Costs now, or down the line? Wheelsets tend to last longer due to the rotor being sacrificial rather than the rim, and will be future proofed better as discs are a more contemporary system. Lots of cheap hardware out there, but it would require disc specific wheel.
You can either use an adapter like the above, or get a frame builder to braze on tabs. Not always recommended but it looks like a strong frame in good condition.
Discs are not something I'm well versed in. How straightforward a job is it?
Also looking to keep the cost as minimal as I can and the maintenance as fuss-free as possible.