well, only real expensive things you might need for your necro would be a nigma and maybe HotO. a cheap build (but really good) would be the following:
-shako (or peasant crown)
-enigma (mandatory if u want to pvp) or vipermagi (for pvm only)
-white wand runeword (dol+io on 2 socket wands, try to find one with +spear, spirit, decrep, amp or revives on it)/wizpike/spirit sword/boneshade/suicide branch, all cheap and great options if you cant afford HotO. if u can, get one nonetheless.
-cheap fcr rings/ravenfrosts
-glooms trap/nightsmoke/string of ears-
-eye of etlich/+2 necro skills magic ammy
-homunculus/spirit (cheap if youre on ladder, expensive in NL )
-anni/torch if can afford
-poison bone skill gcs if can afford
this is a pretty good starter budget.
as for build, go max block with homunculus, rest in vita. just add str for using your armor. if you got torch/anni/enigma, wait until lvl 75 to add points and put on stuff.
skills, i advice these:
-only 1 bone armor
-1 in decrep and prereq
-1 in revives and prereq
-1 in golem mastery
-rest in bone wall.