Great! I'm pretty sure the port was originally designed for the 2/5 (hence the "25" in "LISA25.SYS").
This is one of the reasons I get a strong feeling that this wasn't ever really finished; it was a demo platform, rather than a serious port. Error handling is nonexistent (if you get an abort/retry/ignore prompt and press 'r' for retry, it actually restarts the machine, at least here). I have found so many ways to crash this so far
So it boots off an X/Profile on a 2/5. HELLO.APP works, but 1ST_WORD.APP causes a complete system hang when trying to create a new document. Make sure to have a floppy disk inserted if you try to access the drive, otherwise it will crash the system with Lisa Error 47.
Also: don't forget to copy ASSIGN.SYS and LISA25.SYS to the hard drive when doing your file copies.
Having two Macs is never a bad thing. They're very comparable, as everyone has said. You may want to pick up a pair of LocalTalk or PhoneNet boxes and play around with networking them.
In terms of the Plus vs SE debate, don't forget the Classic is also comparable. Its advantages are RAM expansion being easier (as long as you have the card) since no Classic board used resistors and its screen being easier to adjust without taking the case off. The ROM disk is also a nice perk. However, the Classic is notorious for capacitor problems and has design limitations based on it being the cost-cutting model of the SE (no second floppy capability, only one ADB port, no expansion slot, weaker power supply). The SE wins overall between these three machines.