Buy jewelry first if you want to "save" your plat in your items, that'll hold it's value. At your level, stack HP more than mana. You'll corpse run less. 2x Adamantite Bands = 150hp, which is essentially 7.5 levels of extra hp, for about 300p. Golden Jaded Bracelets will add another 1.5 levels of hp and about a level extra of mana for 60p.
I like Advisor Robe because it adds 20 mana, but also adds 5 Int, so that will continue to help your mana pool in your 30's and 40's and it also looks cool for a cheap robe.
650p will be a drop in the bucket of what you're going to make as a necro, so don't get too hung up on it. A necro can level on spells alone for quite some time anyway.
Right now you can kill wisps in NK and turn in the lightstones and greater lightstones for exp.
http://wiki.project1999.com/Research_Aid The books the greater lightstones get you can be sold right back to the same merchant for 9-10p. Good exp to about 16. If you start now, you'll easily make a few hundred plat just leveling. If you do a little research and plan out a leveling path too, you can easily make 10-15k before level 30 solo while you level.