Alas, I doubt that your scope can be upgraded. If it does not have any sort of electronic controller already with it, then it it not meant to accept electronics. If I understand the model you mean, yours had a single-speed equatorial drive and no electrical hand control at all. If that is not the case, let me know. It is a good scope for the money--good optics--but the small aperture is limiting. When it comes to telescopes, aperture is king. Let me put that in capitals: Aperture is King, Aperture is King,...
The portability of the ETX is its best asset. Plus, it makes a reasonable daytime scope, also--something that is not true of Newtonian scopes.
Tripod stability is important, but if you're going to spend much more money on astronomy, save it for a larger scope.
There is a huge difference among Barlow lenses. The cheapest one worth owning is the Celestron Ultima. The image is only as good as the worst piece of glass in the optical train. A cheap Barlow will make you think all Barlows are trash. The Celestron Ultima, or anything from TeleVue (Barlow or Powermate) will serve you well.
Meade Plossls are OK--you can live with them until you buy a larger scope, and maybe for a long time after that. With the ETX-90, I would opt for the following trio of eyepieces: 32mm (for widest angle, though if your scope came with the 26mm, stay with that), about 18mm for medium power, and about 10mm for highest power. Anything much below 10mm is a waste with that scope. One good Barlow and a 32mm or 26mm eyepiece may really be all you need.
As for filter types, I don't use them much. I would not be surprised if the Meade and Celestron filters were made by the same 3rd-party vendor in Japan or China.
Don't supercharge your scope. You can now buy a brand new ETX-90 non-go-to (called ETX-90RA or ETX-90 Model M) for about $250, and you would spend that much on supercharging.
I personally own an ETX-90, but it now serves as a mere guide scope on my 10-inch LX-200. If you have the money, but a scope of 8 inches or larger aperture, such as the LX-90, LX-200, or even a Dobsonian.
Take a look at Orion Telescopes (at telescope.com ). They have just added computerized "go to" to their Dobsonians without raising price at all--an astonishing feat, if they've done it well. I have no experiene at all with their electronics, because it is such a new product, but their Dobs already had a good reputation in terms of bang-for-the-buck. So consider an 8-inch or 10-inch Orion Dob.