Ok, so I'd ask myself whether I _need_ to autofocus this early in my imaging career. You may find that a Bahtinov mask provides you with all the precision you need.
On whether you need the field flattener - most refractors' field has a radius of about 1/3 of the focal length (not true for Petzvals and some exotics, but it's a fair approximation for the WO FLT series. So, let's guess the field has a radius of 7*132/3 = 308mm.
Field flatness starts to become an issue when the star bloat caused by field curvature exceeds the bloat caused by other factors like the atmosphere - let's say around 2 arcsec for a first pass.
There are 4.8 microradians in an arcsec so a 2 arcsec spot at the focal plane will subtend about 9.6 *10-6 * 7*132 (mm) ~= 9 microns.
So if the cone caused by the field curvature exceeds 9 microns, you need to pay attention to it. Because it's an f/7 scope, this translates to 7*9 = 63 microns However, if you focus optimally, you can probably tolerate 100 microns error before field curvature produces an observable blur.
To estimate the angle that this corresponds to, we can use the relationship
r cos(theta) = r - 100 um or cos(theta) = 1 - (100um/r). For small angles,
Recall that we've calculated the radius of curvature to be 308mm so we can write
0.1/308 = 0.5*theta2 >>>> theta = 0.025 radians = 1.46 degrees.
Since this is half the field, we have that a field diagonal of about 2.9 degrees can be focussed well enough on a 5" f/7 scope for a field flattener to be unnecessary. (That's about a 46mm diagonal, or roughly the size of a 36x24 ccd.)
All you need to know is that to achieve 2 arcsec resolution, you should focus accurately about 2/3 out to the edge.