Ok, so I actually have a character that did this. I created him with the GM to make sure that it wouldn't be twinked (except in doing this it was) and this is what we came up with.
To create the spirit you would have to follow the Ally spirit creation rules in MITS and purchase the Karma as spell points @ 25k a pop (even if mundane) and then create the spirit, which could get you a force 3 spirit with 3 spirit energy and a sorcery of 5. The GM would look at the spells in MITS and choose numbers to corrispond with the spells and make a random roll to see what the spirit would have for spells. My spirit ended up with Manabolt, Mindprobe and control emotion. For powers we gave him only a few, Hidden life obviously, Human form, astral gateway and Guard.
One thing to note making the spirit took a lot of building points out of the character for starting so the attributes of such characters are usually "normal" as in the joe shmoe on the street.
On the mental flaws part, we created a flaw dubbed Perception Jump -5 pts. which the GM would role at the beginning of the session and depending on what the role came out as the flaw could happen during the session at anytime. Perception jump is a flaw that at any time the characters senses (sight, smell, taste, touch, sound) jumps into the entity of the spirit incapacitating the PC's body and appearing as narcolepsy. However, the spirit cannot tell that the PC is seeing through it's own eyes and there is no way to fix it. The way it worked for me was that it so happened that only when i got injured did my PC "jump," this was obviously confusing to me so i was a little more careful about getting hurt.
Anyway, the way that hidden life provided a bonus to the character was excellent. The character recieved +Force to all physical attributes, str, quick, body... NO mental att.s because we determined that it wouldn't make sense, most humans are "smarter" than a lot of spirits in the way of knowledge and since Force doesn't necessarily determine the Mental stats anyway there is no reason to have them affect the PC. Other bonuses were Immunity to age, pathogens, toxins, and normal weapons... which after much searching was found p. 264 Basic under the power Immunity and gives an armor bonus of twice the PC's essence acting like hardened armor. To top that off the PC also got Regeneration.
If you want to raise the force of the spirit you have to give it karma with the same rules as doing so..mundanes at 3 to 1, aspected at 2 to 1, and full mages at 1 to 1. This gets really costly on the karma, but once your physical stats are so high that raising them would be more expensive, you raise the force and instead raise all three.
So in conclusion, what happens when you make a PC with this is you get a character that will mostlikely not die but won't be very good in combat unless the few skills you choose are about 3 all at 6... but oh man is it fun to play.
So if you want to make a character, this has seemed like the best way to do so, the character doesn't seem twinked in that he can be killed by magic or really b****en' characters but overall he just won't die. Which really is what it is all about.