It's funny that they don't give you an audio spec for the instrument i/p, yet do for the mic. Were you trying to capture something from a modern digital source, I'd suggest finding an interface with a dedicated Line i/p, as the 0.5Meg Ohm instrument i/p may not have noise performance that's as low as, say, a 20KR dedicated line i/p.
However...we are talking about cassette here, so whatever noise advantage is offered by a dedicated line i/p will be swamped by cassette's inherent hiss. If the intrument i/p has L/R gain controls, then make sure they are nicely balanced, as it can be easy to make a discrepancy in L/R balance. Record and be happy
NB - the instrument i/p will likely have a lot more gain than is needed for a line source. Try to get as hot a level as possible out of the tape deck, and as little make up gain as you need from soundcard. If deck has an o/p level pot, don't distort the instrument i/p, though!
Modest soundcard = 90dB SNR
Cassette (no Dolby) = ~60dB SNR
If you record with a few dB margin under 0dBFS, don't worry!