Think the 5400rpm in laptops is to keep them cooler and use less power/battery compared to the faster ones.
If you are happy to install Windows yourself, just get a machine and relace the drive. I would expect most i5 machines are new enough to not have issues with SSD and you can always get a usb caddy and use the drive it came with as an external extra drive when/if needed. 128GB seem to be the sweet size to get for SSD for price.
This one is £45 but usually the 64GB are £35, so worth that extra tenner imo.