Used-bike: How much to reserve for repairs/upgrades of total budget?
For convenience here I've made some assumptions here about the type of bike you're buying, but since you don't say...

If I were buying a second-hand bike, I would examine the following as soon as I got it:


chain
cassette
tyres
brake pads
brake/gear cables (conceivable but unlikely)


and if any of these showed sufficient wear I would change them. You can get a ballpark estimate of the associated cost from Google, but note that changing the top two and the last requires special tools, so you might need to factor these in as well (or mechanic costs). 

I'd have thought all those items would come to a bit less than €100 as a guide, just the parts, with no labour or tools. But you should take this with a pinch of salt because it depends on the specific items you get - you could quite easily blow €100 on a pair of tyres alone if the fancy took you. Similarly you probably wouldn't have to change all of them, at least initially.

Provided the bike is in generally good condition, this should be all you need to change in the  short term (first year or so). But of course because it is used, you're taking a bit of a chance that it doesn't have other stuff wrong with it that you'll need to sort.