Yes you can force a RPi2 or 3 to run a node but it is going to be a slow miserable time and you will have lots of issues.
Raspberry Pi4 is much better option because they have much bigger option to choose more RAM, that is if yo can find them available in stock
Yes you can download the entire blockchain and run a node on a 500GB drive. But you WILL run out of space and then probably have corruption and then have to buy a larger drive and start again. And you might not even be able to copy the data from the old drive if the corruption was bad enough.
Unless you are running pruned node, that can run on even smaller hard disk without any issues.
I don't think larger hard drives are problem for anyone nowadays, old style HDD are very cheap now and you can find them anywhere.
Yes you can use older equipment to do it. You can even use 10 year old laptops with a 2nd gen core i5. But much older then that the pure performance issues are going to be a killer. And as time goes on it's only going to get worse.
If you look at ebay or craiglist you can find very cheap good laptops with more than enough RAM, processor and hard disk.
I would say that for around $100 you can get great used laptop with 3rd or 4th generation i5 or i7 intel with SSD and 8gb RAM, but better look models that can be upgraded in future, with socketed parts, not soldered.
but only because 1TB SSDs are still fairly expensive.
I don't think they are so much expensive anymore, you can quickly find very good new 1TB for around $80 to $90, and even cheaper on some deals.
There are a bunch of used 100% health 2.5 inch SSD drives being sold online, so you can find good Samsung 860 evo for around $60 to $70.
However, you should always look for TBW and higher number means that disk will last much longer, for 860 evo that would be 600TB.