I figure you could buy an entry level 8 string from Schecter and be done with it.
Couple thoughts:
The lowest string on an 8 string is in bass guitar territory, I often find I avoid that string ( f# ), since I tend to play the same octave on bass, as even a 5 string bass does not have an octave lower equivalent. If the music calls for the guitar to be an octave or more above the bass guitar, that is.
The scale on my 8 string is similar to a 6 string, and I find the tension normal to high on the higher six strings, but a bit floppy on the lower two, especially the lowest string.
A seven string guitar makes alot of sense, and the low B is really useful. The lowest string ( F# ) seems to want a different amp setting, and while I sometimes use notes below B, I don’t go too far below.
You gotta adjust to playing an 8 string, and the lower strings being heavier gauge require an adjustment, especially to keep the tuning accurate.
Clean amp settings and playing, however are another thing altogether, and the low F# string and the resulting chords available with the 8 strings brings a whole new sonic territory to your musical arsenal. For that reason, I suggest making sure the pickups can give you decent clean sounds. I strangely prefer passive pickups to active ones, especially EMG’s. The EMG’s are great for distortion but I felt the passive had more tonal ranges, even if not as hot output.
An actual baritone guitar with B as the lowest note will have a longer scale, and this is also an option to an 8 string. You could experiment with string gauges that get you that low F# and see if the tension is better than a shorter scale 8 string guitar. Assuming that lowest F# is what you are after. There is also that fender bass that could do the same thing.
Finally, as a writing tool, an 8 string guitar is a great tool to have, even an entry level new guitar.