I don't know how much the ghost heals. Joram stated in another topic somewhere that it is helpful throughout the game if you keep it upgraded. At least early in the game the ghost is the only healing spell you have access to. In the original release of D2:ED and in DKS I was fine with food and potions early in the game (though trying to conserve potions could mean sometimes standing around for awhile waiting to regenerate), and once I hit level 15 I learned the Healing spell (never needed to boost it past level 1).
From Lancet's DKS FAQ/Guide:
SUMMON GHOST: This is like Summon Undead, except with a much longer cooldown,
less duration, and the chance to cast Healing on you. It usually sits around,
lobbing energy balls that do much, much less than the Undead's version. The
Healing spell can make it useful, especially early on, so a couple of points
into this skill at the start may pay off.
Level 1
Continuous skill
Cooldown: 90 seconds
Stats 1 2 3 4 5 6 7 8 9 10
---------------------------------------------------------------------------
Summon level 1 4 9 11 14 17 19 22 24 27
Duration 60 70 80 90 100 110 120 130 140 150
Mana cost 22 34 54 62 74 86 94 106 114 126
11 12 13 14 15
--------------------------------
30 32 35 37 40 Summon level
160 170 180 190 200 Duration
138 146 158 166 178 Mana cost
I used the Mining Guild set with my ranger for quite awhile (one of the level 15 sets you start with in the Steam version and North American Xbox version), but other than that just used random, unique and purchased equipment. I did collect set pieces, though, and saved to try out a couple of them (reloading being easier than re-equipping all the original equipment).
