I may be able to buy a TDS5104B for under $1,000. Does anyone have experience with this scope (or the TDS5000 family)?
Yes, one of our labs had them when they were new, and even back then they weren't something to write home about. The positives are the large screen and the decent UI (much better than the UI on Agilent's Infiniium 54800 Series scopes), however like most Tek DSOs they were very slow in general, and much more so when trying to do anything demanding, a point at which they tend to lock up. They were also pretty crash-prone when under heavy load.
Spec-wise the TDS5k Series OK (low sample rates for this class of scopes, which goes down from 5GSa/s in single channel to merely 1.25GSa/s in 4ch mode), aside from the price (which was Tek typcial on the upper end of the spectrum).
These scopes also have a mode called 'Fast-Acq' which increases the waveform rate, however measurements and analysis tools don't work in this mode which makes it of little use, als the claimed 100k wfms/s in that mode are reached by other scopes without the measurement/analysis limitations.
Also, if I remember correctly, they also only ever ran Windows 2000, not XP, even at a time when all competitors had their instruments solidly on XP. Unlike other Windows scopes the TDS5k uses a proprietary mainboard format, and some of these boards suffered from leaking caps.
These days I wouldn't buy them. They were mediocre back then and are poor scopes today. Tek made the best analog scopes, but in terms of DSOs they pretty much only had poor to mediocre products, and the TDS5k is no exception. Plus they have been obsolete for a long time, and seriously, for the money a TDS5k often sells for (i.e. >$2k for a TDS5054B and >$3k for a TDS5104B) you can get much better scopes on the 2nd hand market.
So unless you get a really good price (i.e. less than $1k for a fully working TDS5054B) or you're specifically into Tek gear for some reason then you're better off with a different scope from other brands.