Associated does make some good products. My first hobby grade truck was an Associated RC-18t so I have had experience with them. I had lots of problems with their electronics.
I would not recommend a nitro powered truck as a first vehicle. Try the electric version first so you can get used to driving/wrenching on it then go to nitro. Nitro adds another layer of complexity to the mix. There are electric versions of the trucks you mentioned.
I like nitro better than electric. I think it's cooler. I know it's not easy to use them but I guess that the whole part of this hobby. To learn and get better.
i have a rustler, think its the next model up from the sport, which is a great wee yoke. goes like stink and is a blast.
only problem is the traxxas engines (i find) are a bollocks to tune and to get set up right. dont get me wrong, once they are tuned sweet they wil blast all day long.
as for parts, the closest hobbie shop to me is over 30 miles away so i alway have to order online but the is never a shortage of parts