The prices I've seen are DR-03 for $59, while all three of the other models (PR-10, DR-05, and DR-07) are $99 (those are the original versions, not the MkII). The DR-03 and PR-10 have the advantage of smaller size, while the other two models MAY have superior pres and ADCs. I think street prices of the MkII versions of the 05 and 07 are closer to $150, which is approaching Sony territory.
So I would just grab the DR-03 for $59, unless one of the other $99 models has significantly superior pre and ADC. With a big company like Tascam, I would not be surprised to find that all four models have exactly the same hardware. I can't find detailed specifications on Tascam's web site. It's not like 25 years ago when the published all the data. Heck, I remember when Tascam provided a full schematic with every piece of gear they sold!
An external pre is out of the question. I want to go smaller and less cumbersome, not more so. For my purposes, the MTII through the 1/8" mic input has always been satisfactory in the past, so perhaps even the $59 DR-03 would be a step up from the MTII? Then again, would there be any advantage to spending the extra $40 on the DR-07 while they're still available at retail for $99?