The current M109 variant in use by the Regular Army is the M109A6 Paladin produced by both Italeri and a rebox by Revell. It is a good kit, but the tracks are so-so. Fortunately Skybow (now AFV Club) make a set of single link tracks to replace them.
The MNARNG may use an earlier version of the M109, possibly the M109A5. Italeri makes (made?) a version of the M109 that builds into all versions between the M109 through M109A4. It is kit #246 and called M109E4/G/A1-A2/E4.
The physical difference between an M109A4 and M109A5 are minor (A5 has a longer tube) and would be difficult to point out.
As far as an FDC, they ride around in an M577A2 command track carrier. Tamiya makes an older variant of this (M577) and it could be kit bashed with an Academy M113A2 to update it to a more recent version.