The 424mkII tapes are compatible with the new 424mkIII.
In general, the 424mkII can't be found new, and niether can the 488mkII, so a brand new 488mkII might be the nice score. However, even though the 488mkII is a great machine, cassette 8 track may not be everyone's favorite format, due to the extremely narrow track width. Also, the 488's heads won't be a 1:1 track match to the 424mkII's tapes, but it will be fairly close.
A decent used 424mkII can be found for about $220, so if you're on a budget that won't allow a new 424mkIII, the used mkII is the way to go.
If you want the best new 4 track Portastudio that will be most compatible with your older tapes, the 424mkIII is the way to go, currently being $329, new at GC.