Hi! After 3 years I have not needed any wear replacements apart from the usual: Brake pads, chain, and eventually tyres. As discussed a few posts back the Birdy has some replacement requirements for handlebar, stem and front swingarm, see that post. You should find out how far the previous Birdy has been ridden, and consider if you trust the number. If unsure, you may want to replace those parts, at least if you suspect it may have gone farther than 10 000 km. If there is any indication it has been involved in a hard crash, don't buy.

Some considerations:
Bigger wheels may be nice, but the Helix is unsuspended, right? The Birdy has full suspension. That's crucial for me. The Helix seems also to be made for fairly narrow tyres, with 40 mm the biggest option, unknown if that is mudguard compatible. Birdy can fit 50 mm semi-balloon tyres with mudguards for a very comfortable ride.
Bigger wheels make it easier to get fast gearing.
24" wheels means a small selection of available tyres. That's more or less the same for the Birdy at 18".
The one legged Helix fork makes it hard or impossible to find any hub dynamo solution for it. The Birdy has this convenient option (though it requires a special hub).