The slimmest Phicen body, M31, in my opinion, is not worth it (I know not all will agree) if any part of the body will remain uncovered, as in this instance. Yes, you will have excellent articulation (except if/where restricted by armor or clothing). But those seamless arms you will get will look a bit like noodles at the elbows when you flex them and you will have problems with the wrists if you ever change the hands.
Body: Go with at least M32 (although this is a tad slim on the lower arms and lower legs), better yet M33 (if you want it taller, it comes with specially designed ankle extenders, which are fairly seamless and at any rate would be largely obscured by the boots and greaves -- speaking of the boots, you will need to keep the ACI feet because the Phicen feet will probably not fit in them. ACI and World Box feet work on Phicen.).
On M31, M32 and M33, see my reviews
HERE,
HERE, and
HERE. For M34 you can see
HERE, but it might be too muscular for the armor and clothing in this instance.
Armor: I excavated the figure and looked it over to answer your question. Looks like you will have to untie several items like the kilt, leather tunic, and maybe greaves. Unless you discover their tied ends, you will have to cut through the little lacing on each side of the armor (under the armpit). Then gently (!) open up the armor and slide it up and off the body. Heating it might help, but be careful. After you get it on your Phicen you will need to replace the the lacing under the armpits (actually I don't know, you might be able to get away without it). This should be it, but again all this is hypothetical, based on looking at the figure and minimal futzing with it. I haven't actually done what you are planning to do.
Hope this helps.