Honestly...I would go for:
Cleric 5/Summoner 10 (from Relics and Rituals)+whatever
The summoner prestige class will let you gain 1 extra summoning spell, and it will be of your highest possible summoning +1, so if you were a Cleric 5/Summoner 4, you could cast up to Summon Monster V, with the extra summoning slot you could summon with the Summon Monster VI spell. These creatures however must be of your patron's type. (read more in Relics and Rituals)
Also, the summoner PrC grants you up to 5 metamagic feats only usable on summoning spells, and it will let you use up to 5 of these feats to enhance a summoning spell (1 feat at a time) without raising its spell level. So that Cleric 5/Summoner 4 could summon something from the Summon Monster VI table, and make it Twinned (FRCS I think, either that or T&B) or Repeated (T&B) for free
If you can talk your DM into getting that summoning prestige domain at level 1, go for it, otherwise get that 1 level in contemplative.
Augment Summoning is a must-have feat of course.
One of these two items, the first prefered...
Ring of the Blue Conjurer from the Book of Eldritch Might (I don't know which one)
blue conjurer gives summoned critter +4 def ac bonus, +4 to con (effectively +2 fort saves, +2/hd hp) +2 bonus to strength (effective +1 to melee attack and damage rolls)
Price-42k
Ring of the Red Conjurer: +2 defl ac, +2 con to all summoned creatures
Price-14k
Spell Thematics from Magic of Faerun (errata'd one), to all of your summon monster spells, so that they are +1 caster level (net +2 with summoning) and they can look even cooler!
Item with Bless activatable at will, price is only 2k!
Spell Girding (Magic of Faerun), gives you a +2 against dispel checks so that your summoned creatures are harder to dispel.
Edit-Be LN or similar and summon Element animals instead of fiendish/celestial mostly, these get past that alignment restriction since most of the creatures are turned into N alignment.