The Jensen transformer upgrade is meant for the mic preamps. Some folks really like the coloration that transformers give to condenser mics (gives them more of a ballsy sound,.. read classic rock). They could provide some galvanic isolation but I've never had a noise issue with my MIO so I can't see that being worth the extra $$$.
If you're looking for a used ULN-2, the price will vary quite a bit depending on how it's configured.
I've seen the basic unit go for as little as $600 but you'll probably have a hard time finding one without either the " plus DSP", "2D card" or both upgrades. Both options are for processing audio, advanced routing, mixing, and recording. For someone who wants to use it exclusively for the DAC and maybe some routing don't really need either.
If you feel like you might be interested in doing needle drops with digital RIAA eq, you'll need +DSP.
Be aware that you can add any of these options anytime you wish.
I bought my ULN-2+DSP in 2007 for $800 just a month before the 2D card was introduced and I didn't get around to adding it till last year.
I think you'll have a tough time finding a ULN-2 without the card but you should be able to find a 2D unit without a DSP licence.