Yes, that is a tough question to answer – and ultimately only you can answer …
My thinking is that a set of 7 harps in different keys sounds interesting, but really only if you play in a band.
With two harps of better quality you can still play in several different keys that are popular in rock/blues music (e.g. a C harp and an A harp: with the C harp you can play along with most rock or blues tunes that are in the keys of G/G-minor, D minor, E minor and A minor besides C; and with the A harp you can play along with tunes in the keys of E/E-minor, B minor and A). Other possibilities are also there with these 2 harps but require either more advanced techniques or are for songs in less common keys, such as C# minor using the A harp.
In place of the A harp, other popular harp keys are the G harp (for music in G, D/D-minor, A minor, B minor, and E minor) or the Bb harp (music in Bb, F/F-minor, C minor, D minor and G minor).
Another thing to consider: If the more expensive harps have stainless steel reeds then they typically last much longer than simple brass reeds.
A tough question, indeed …