I was going by the other posts in this thread which stated a frequent maintenance schedule often replacing expensive parts and the general warnings elsewhere which admittedly may be the early version. However from what you have stated it sounds like you have made quite a lot of investment in the bike to improve it. I had in mind people that buy a bike and with very simple maintenance pretty much do nothing else to the bike and can use it for many years and when a part does fail it is so widely available that they can get it at a good price. In addition to the frame and suspension parts and possibly issues with having 18" wheels I thought there might have been a few more specific parts admittedly especially around keeping the chain tensioned. However I don't know the cost of the specific parts or their frequency of replacement to gauge cost myself anyway and can only go with by what other's have written forums.

On the bike of this thread is it 7 speed with a cassette if so is that cassette hub easy to repair/maintain I thought such a hub might be quite a specialist part nowadays again not applicable to later Birdy's I'm sure. I think on some of the Dahon's some older models had a 7 speed cassette and the hub was difficult to repair/replace and later entry level Dahon's just downgraded to a tourney freewheel which while not as robust or reliable at least is easily replaced.