Best projection for linear transformations

Dear all,

I am encountering a problem and I could not find any clear information
on Internet.
Basicaly,I have geodesic files which coordinates are geographic (lat/
long) and which datum is WGS84.
I need to perform some (linear) geometric operations on them
(translation,rotations... = Lat/Long to XY ,transform,then XY to Lat/
Long),and I could not really understand what projection system to
use.At first,I used UTM (That worked well),but I had problems when
data was overlapping more than one zone.I know that projections are
valid more or less locally,but I need to have advices on projections,
that I can use to perform linear operations with less restrictions
than UTM (what happend if you rotate two points that are in different
zones?(distortion)).My only goal is to have coherent results (lat/
long).I will agree also with solutions with well defined limitations
(No rotation for extents of more than xxx....).

Thank you in advance.
Best regards.

N.