I guess samba might do the job, but there are also NFS-servers for Windows NT. I'm not sure how they compare performance-wise, though.
However, if the cost of storage is an issue, why not use 'cheap' Intel-based PC's and run a free *nix dialect, such as Linux or a BSD-derivative. Then you still run Unix on both environments, and have bultin NFS-support, telnet et al. Use a journaling filesystem such as jfs (from IBM) or xfs (originally from SGI, I believe), and partition size & file-handling will be a non-issue.
If I'm not misinformed, SUN actually has endorsed Linux - which could be beneficial in this case.
Rgds.