Aldi/lid or the same on ebay are perfectly fine for the old bike. Seatpost vary by 0.1mm anyway even between the same size.
They are pretty repeatable and accurate for the intended purpose.
Take the battery out when not used for a bit.
Keep the run clean as oil can make the measurements jump.
I have two sets at home and we use them for our a-level and igcse pupils at work and they've not broken them yet.
You end up using them for all sorts after a bit if they're just lying there.
For thin stuff though, screw gauge work better, cannot think of anything on a bike that would need that as even the callipers will pick out the v-brake shim kit widths.
Oh and buy SR44 (silver oxide) over LR44 (alkaline) for them. EBay is a good source... I should know as I buy loads for these damn Hexbugs