OK, I'll try to help.
First off, with
40k, you really should focus on whichever army looks most interesting to you. Despite the noise,
40k armies are (relatively) well-balanced with respect to one another, so choosing one army will not give you massive metagame advantage / disadvantage. Thus, you aren't likely to be especially unhappy with any of your listed choices.
As you have and Orks, and are looking for a 3rd:
- Tau are a good choice that is different and plays differently. Most of the core models (Fire Warriors, Battlesuits, Tanks) are plastic, so it's not too bad on the budget.
- Necrons are very much like
SM, and their Codex is very old, and not so much to play against. I'd suggest you avoid this army in favor of one of the others.
- Eldar have a newish Codex, a 4.5 Codex, if you will. It's pretty decent, especially as a new player. But it will want a lot of expensive models, so not a good choice for a budget player. The high specialization is an interesting play style.
- Tyranids have an older Codex, but are still strong. I would suggest this as they are a classic nemesis for
SM and Orks.
So in summary, I would rank like this:
1. Tyranids
2. Eldar
3. Tau
4. Necrons
-- $k 
-- 9k 
-- 6k 
-- 4k 
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