Early 90/110 brake pedal assemblies with the type 50 servo will also bolt in place if you can't get a good series 3 one. I put a Defender one in my current project just yesterday to go with a disc conversion, I did have to rebend the pedal to straighten it up as it is bent to the right on a Defender not leaving much room in my footwell.
Later type 80 servos are much bigger and you'll probably have difficulty making one fit in a 2A. It's worth having a read of this: http://www.expeditionlandrover.info/dual_brake_conv.htm
As everyone else has said it's worth stripping down and refurbing your current arrangement, and making sure it's all properly adjusted before you decide to change too much as a lot of folk are happy with the standard.
If you decide to go even further then there are a few disc conversions available. The Zeus kit is the cheapest but uses E-Type calipers and machined discs and won't take standard wheels, the Rocky Mountain kit uses GM parts and machined discs and again won't take standard wheels, the Heystee & Roam kits are the best as they use replacement swivel housings and hubs designed to take standard Defender parts but they are also the most expensive. I went for the Heystee kit as after reading the problems people are now having getting spares for the old Chris Perfect systems I decided getting a kit that uses standard LR spares would be best as I plan to keep the vehicle for a long time. It will also allow the use of standard rims and looking at it built up I think I'd even be ok with 15" rims if I wanted to. Esentially when I've finished I will have an early Defender brake system in my Series 3 but it would of been cheaper to buy an early Defender...