I would think you won't have a problem with a F4 scope as long as you use good eyepieces and I don't think $20 eyepieces will give you sharp views. It all depends on how much detail you want to observe, how polluted your skies are and how steady your mount is.

I would suggest obtaining a 2x or 3x barlow and then you don't need as many eyepieces. A 12.5 eyepiece and a 2X barlow will get you a 6.25 eyepiece view and there's always room for a 40 mm eyepiece too which gives you a wider view for nebulas and star clusters.

A good place to find used eyepieces and everything else is Astromart Classifieds

Oh, 25 mm eyepiece at 36 magnification means you only have a 900 mm focal length?

Hope that helps and clear skies!