Unlikely to fit 650B, they are only an inch bigger than 26" but there's not usually that much spare room at the back of MTB frames.
You should be able to finish it for ~€1000 with mid-range kit
€200 wheels €100 tyres €200 gears kit €100 crankset €200 brakes €100 rigid fork €100 bars/stem/seat post
That just leaves you short of saddle and pedals, but there's enough wiggle room in the list above to make up the gap, without dropping out of the usable gear into the worthless crap.
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Unlikely to fit 650B, they are only an inch bigger than 26" but there's not usually that much spare room at the back of MTB frames.
You should be able to finish it for ~€1000 with mid-range kit
€200 wheels
€100 tyres
€200 gears kit
€100 crankset
€200 brakes
€100 rigid fork
€100 bars/stem/seat post
That just leaves you short of saddle and pedals, but there's enough wiggle room in the list above to make up the gap, without dropping out of the usable gear into the worthless crap.