Three hundred gold? That will buy you... a breastplate, maybe some banded mail. A level 20 character would be expected to have about 880,000 gold by that level if they didn't buy anything terribly expensive.
It's rather difficult to raise saves with equipment beyond what you've already mentioned. I would recommend looking through the Wondrous Items list for the things you are looking for. Important items could be the Bracers of Armor or Ring of Deflection
I hope you have more than 300 gold. That could buy you a nice suit of armor, but not much else. As for defensive items, consider stat boosts for Dex, Con, and Wis to increase the related saves. You can also purchase a mantle of faith to gain DR 5/evil, periapts that make you immune to poison or disease, and a scarab of protection can give you SR 20 and immunity to the next 12 energy draining attacks, death effects, or negative energy effects.
What class is this for? This will vary the answer since a monk may love bracers of armour +5, but it's useless on any character who already wears a significant amount of armour.
Another way to go for increasing saves is to increase their respective ability scores. Raising Dex, Wis, or Con will increase your reflex, will or fortitude saves respectively. Grabbing a few +4 or +6 items to these will set you back around 64k each
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I can not talk about the class, it have to be secret.
And yes, I will buy +6 items for dex, con and wis.
Well, I will conclude that the character is not heavily armored as the character would not benefit from a high AC boost to dexterity while wearing full plate.
As such, I would recommend bracers of armor class (decide the value, but +8 should work), a ring of deflection +5, an amulet of natural armor and perhaps see if you can get some custom made items. (Combining a cloak of displacement with a cloak of protection might be nice.)
Also, having some way to avoid falling damage is a good idea. A ring of air elemental command can function as a ring of feather falling.
My quick advice would be to use this guide as a starting off point: http://www.giantitp.com/forums/showthread.php?t=187851
It's 3.5, but it should give you a good idea of what areas you should cover. The new magic item pricing rules are also similar in 3.5 and pathfinder, so if the DM is letting you use them you can get some decent custom stuff. Sorry I don't have any specific suggestions though.