Feel free to buy that Lumia 535, there is no end date to the free upgrade to Windows 10 Mobile.
Unfortunately I do not have a source to back this up, mostly because Microsoft has never, not once, so much as hinted at charging money for the upgrade. Just like they never charged money for the upgrade to, say, WP 7.5 or 8.1. And just like no other mobile OS vendor (at least in recent history) has charged money for an OS upgrade. When you think about it, that in itself is all the guarantee you need that the Windows 10 Mobile upgrade will remain free of charge - were Microsoft to make it a paid upgrade, they would completely alienate pretty much all of their remaining users and cause massive uproar.
The only way the upgrade can end up costing you money is if you have to download it over a mobile data connection (but it'd have to be another device's hotspot, because downloading W10M requires a WiFi connection).