Not easy to get absolutely all we want in one machine ... I've also noted very good pricing for the 18 in sit-down Tin Lizzie, and also for the Janome Artistic Design 18 in. sit-down. As a matter, they look very much alike. Probably same manufacturer?
Regarding the harp depth (height), my Juki has a harp space of 9 in. x 6 in. high, so the Tin Lizzie or the Janome would give me double the harp space. So I guess, Mama's place, you need to compare the harp space you presently have with the harp space you'd be getting with the sit-down Tin Lizzie.
Let us know what you decide.