Yeah an 1841 would be ideal, and there have been some relative ebay bargains that I've missed by the skin of my teeth (most notable was a few months ago when an 1841 went for £220!).
That's my plan, my "lab" is growing at an alarming rate though, but my 2651XM will become part of my voice lab, given it now has an NM-HDV, PSTN card (and I have a PRI-ISDN card for if I fancy routing voice over ISDN), 3x PVDM-12 modules, a full compliment of RAM and Flash, and two WIC1-ADSL cards. It has a nice IPvoice IOS on it, and callmanager expressCouple that to my 4 3524 PoE switches, my 2 3548XL's and my 871, I've got a pretty nice lab environment.
It's a hefty 2651XM![]()
Then again I guess you're not actually not going to put any serious load on it in a lab...