Can I be reasonably certain that a 32-bit OS will still work on a modern desktop?
Yes. Almost all 64-bit capable processors support both 64 bit mode or 32 bit mode.
(Exceptions might be early Itaniums, IBM power CPUs etc, but nothing mainstream.)
Assuming that I will not be doing anything memory intensive, is there any other reason why I should use 64-bit instead?
Yes, there is. In 32 bit mode you can address up to 4GiB of memory. That is up to 4GiB virtual address space. If you fill all of that with physical memory than you no longer benefit from proper address space randomization. That might not be a big problem on systems with 128MiB or similar low memory, but you are sacrificing some security by using 32-bit mode and lots of memory.
You might also miss some features which are only available in 64-bit mode. An example of that which might impact security is the NX bit. This is not in all 32 bit CPUs. Though this has more to do with the age of those CPUs than with 64 or 32-bit mode.
Update 3 years later (2018). We are now reaching the point where the next version of EFI will be 64 bit only. That might change things.