Assuming you have the skills, and a place to work like a garage in your home, the investment need not be too large. You need only the fairly basic tools that are needed for 90% of the work, and buy the rest as needed, probably financing them from those repairs themselves. Likewise, start with a thin stock of the most common, universal parts like brake and gear cables, spokes in the most common lengths, basic pedals, most common disc pads.
When accepting repairs that require stuff you don't have, quote a lead time long enough to buy what's needed.
As you use your stock, replace it and add extras to deepen or broaden the selection as needed. Likewise with tools, as you make a few Euros you can reinvest them to expand tour capabilities, but do not fall into the trap of trying to be able to do everything.
Assuming this is strictly a sideline, and you don't need the added cash to live daily, you can defer taking it, and build thee business off the sweat equity of the first few months, then slowly begin to withdraw as the immediate needs taper off.
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