What needle valve are you currently using?
Are they expensive & how difficult are they to install.
Depends on whether you can find good deals or not; there are sometimes good deals on eBay, but you have to be patient. They are relatively easy to install if you have the correct tools.
Are there some regulators that cannot be upgraded this way?
Some commercially available regulators use products (such as Loctite) that will essentially glue components together. These can be extremely difficult to remove.
My budget is low & I already have put way too much money already than expected so if it costs more than 100 bucks I will pass on it, because I have a 55 sitting an waiting for pressurized CO2 right now , but still saving & planning for the set up. Suggestions would be great.
A Fabco NV-55-18 runs around $40, brand new, I believe. You could get the Ideal 52-1-11 for around $75-80, brand new. Alternatively, as I mentioned previously, searching eBay will sometimes yield much cheaper results.