Golems do have Spell Immunity but this is not a Golem, it's just a construct. You can cast any buffs you want on it. Unless you are talking about the Pathfinder version.
As said, a mithral chain shirt +1 and a heavy mithral shield +1 would only cost a little over 4k gp but would increace AC by +8. The Skillfull enchantment is +2 and can be found in Complete Arcane. A skillful reach weapon, not necessarily the Spiked Chain, would give the Shield Guardian 20' reach which would help keep him out of harm's way.
Constructs still have a version of spell immunity, in many ways more extensive than that given to Golems.
Specifically, they can't be affected by any spell that can be blocked by Spell Resistance.
Basic buffs like
Bear's Endurance are marked as "Spell Resistance: Yes".
The same is true for
Bless and it's relatives.
There isn't an
Enlarge spell that applies to Constructs, even if the SR rule didn't apply.
Even
Endure Elements allows for spell resistance.
You can't protect a Construct with
Feather Fall, nor grant it mobility with
Fly, though it can be
Levitated, if it happens to be within the weight limits.
He can be trapped in an
Entangle (no SR applies) but can't be released with
Freedom of Movement.
He can't be
Hasted, but if the Construct could somehow eat it could benefit from a
Heroes' Feast. Except that Constructs don't eat, of course.
It can't be rendered
Invisible, though it can be
Silenced (in the area, but not affixed to them unless they're willing. That one's weird.)
Not even
Wish,
Miracle or
Limited Wish will help or improve it.
No
Protection or
Resist energy spells will protect it either.
On the other hand, anything you actually could get to work on it could be made
Permanent.
You can't transport Constructs via
Shadow Walk, but they can be
Teleported or carried on a
Floating Disk. Not a
Teleport Circle though.
It's an odd assortment, but for the most part, buffs don't work on them.
I have absolutely no idea whether or not buffing items, like a Ring of Fire Resistance will work for a Construct either. The spells don't work, but items are different from spells, so I don't know.