As many as appeals to your aquascape
Some people prefer a densely planted jungle look, others enjoy a sparse zen. It just depends on what you like. Also remember your plants will fill out, so research how big they get first.
As for lights, I didn't mean get a whole new set up, just a new light bulb! Unless you have LED's it's an easy cheaper switch to improve lighting. -and lets say yours is unreasonably expensive at $10
If you find the spare money laying around blasting sand is $8 for 50lbs. Super cheap and beautiful.
Black Diamond Medium Blasting Abrasives - Tractor Supply Co.
Play sand is even cheaper $5 for 50lbs.
Quikrete® Premium Play Sand®, 50 lb. - Tractor Supply Co.
And 50lbs is more than you will need! Dirt is free. Just something to consider!
So lets see
Light bulbs $10
Sand $5
Dirt $0
Plants 5@ $5 a piece [average], (unless you find a good deal or trade)
Remember you can always start slow. Get a couple, mix them in with your fake plants. My grow out tank is actually half-real half-fake. Looks good too. My local walmart (surprise! surprise!) actually has better plants than the chain stores (and often my local pet store), so shop around! Sometimes if you're lucky you can get multiple plants out of one pot (I got 2 great swords that way!) And java moss well spread, so get a little and eventually you'll have a lot!
Good luck and remember to have fun with it!