What HP calculator should I buy

[grsbanks]

I can echo the recommendation of the HP-42S as I used one for years (still have it). It took a beating over the years and the paint has flaked off the front plate in some areas

However, don't discount the HP-48G/S or HP 50g just yet. Their RPL may be a bit more cumbersome than what the HP-35S uses but it's worth it. Plus, you can write your stuff on a desktop computer, test/debug it with an emulator and then transfer it to a physical machine once you're satisfied that it's up to scratch.

Even though the HP 50g is technically an emulator itself (an ARM system emulating a Saturn chip) it is significantly faster than the HP-48 series. I have a 48GX and a 50g myself. Another big advantage of the 50g over the 48: it can use a bog standard SD card (up to 1GB) for data storage and transfer. Also, it connects to a desktop/laptop PC via USB (you need to deactivate driver signing enforcement to install the drivers on machines running Windows 8.1 and later) as opposed to the 48's serial connection (how many computers have serial ports these days?).

The 50g doesn't seem to suffer from the "faulty keyboard connection" problem that affects many ageing 48s, mine included.

Look for Emu48+ on the internet. This will allow you to test drive the machines, both 48 and 50g, before you take the plunge and make an informed choice. The user manuals for these machines are widely available on the internet.

Also, there's Free42 for you to look at. It's a great emulator of the HP-42S that doesn't actually need the ROM of the machine to function. Emu48+ does need the ROMs.

Hopefully these things will allow you to make your choice.

One final thought... The HP-42S and HP-48G/S might be a lot harder to come by than an HP 50g.