Having a marimba at home makes, in my opinion, the second biggest impact on your child's percussive level of success, second only to enrolling in regular percussion lessons with a skilled teacher.
From the long-term perspective, buying is typically cheaper than renting -- because when your child is done with the marimba, you can turn around and sell it and get most (or possibly all, if you buy used) of your money back. However, that is a big investment that not everybody is willing/able to make, so renting becomes a viable alternative.
In terms of the viable options, we first have to talk marimba ranges:
5-octave is professional grade (though, more and more high school students are using these far more regularly as percussion education continues to advance at a breakneck speed, so it's not unreasonable to consider this.....eventually). With that said, 5-octaves are prohibitively expensive for most students. This is because 5oct marimbas are almost exclusively made with ROSEWOOD (pro level) bars, which doubles the price compared to a synthetic bar equivalent and triples the price of the cheaper padauk wood bars --- if you can even FIND a synthetic or padauk instrument (as they are relatively RARE to find as a 5oct).
For HIGH SCHOOL students' home instrument, 4.3oct is typically considered the optimal balance between range and price. Even for those kids playing 5oct rep, they can learn the notes on a 4.3oct (or even a 4oct) at home and then make the adjustments on the 5oct at their school.
For MIDDLE SCHOOL students, most can actually get away with something smaller (like a 3oct marimba) because the MS repertoire doesn't typically require even a 4.3. (...though, some advanced 8th graders may start itching for the 4.3.)
For 8TH GRADERS, they are near the transition from MS to HS, so it's worth considering both options. (Think MS for short-term and HS for long-term.)
If you're BUYING, then I'd recommend thinking long-term and go with a 4.3oct. I also always recommend to buy USED --- because you can typically find a nice 4.3 padauk marimba for about $2,500 and then, when your percussionist graduates and you want to sell it, you can sell it for about $2,500 and get most/all of your money back. (Note: Until just recently, I used to suggest $2,000 instead of $2,500. However, with supply change issues and inflation over the course of 2023, an updated price seemed appropriate.)
If you're RENTING, I would suggest renting something like a 3oct marimba for MS students and a 4oct or 4.3oct for HS students.
Now, if you want to BUY a smaller instrument for MS and eventually buy something bigger later on, it is the same thinking as buying a 4.3 --- except that you would have the extra step of selling that smaller instrument when it comes time to upgrading to the bigger instrument. That price would be somewhere around $1,500ish (though, this range can vary greatly). It's a viable option, but it's also more work and effort.