The main thing to look for is support for the USB Mass Storage driver
(UMS as it's called on the iRiver website)... that shold be most products, but some companies are starting to replace UMS with Microsoft's MTP (Media Transfer Protocol
50.00 what? Euros? US Dollars? Yuan?
I guess I would suggest anything that uses an SD/MMC card, so you can upgrade your storage as finances become available. Also, a cheap USB Media Card Reader
to grab the recordings off of. (I got one for something like CAN $10.00)