Having owned a lot of the gear that Audio Research built in the 1970's and early 1980's I can speak from experience....
Pass on the D-100, D-110, D-111, and D-350 solid state amplifiers as they were all early efforts into solid state design from Audio Research...and the sound is disapointing. Later Audio Research solid state amplifiers built in the late 1980's and 1990's were much better, such as the D-130, D-200, D-300 and D-400.
The D-79 and D-150 which were originally developed in the 1970's are classic/collectable Audio Research designs and hold up fairly well when compared to later units. I always felt the D-50, D-51 D-75, D-76, D-90, and D-160 had that "soft tube" sound and lack the high definition of the D-70, D-115 and D-250 series.
I can strongly recommend the D-70, D-115, D-250 and M-100 tube amplifiers, as they were a huge improvement over all previous ARC tube designs. That series of amplifiers, featured new transformer designs which gave them better frequency response at both ends.
In terms of preamps...I have owned a couple of SP-3 and SP-2 preamps and when compared to the SP-6 series or SP-8 series they sound pretty dated. The SP-6 and SP-8 offered a large improvement in definition over the earlier ARC preamps...the SP-3 and SP-2 were very soft sounding in much the same way as a Marantz 7 preamp sounds. The solid state preamps that ARC built during that time, such as the SP-4, SP-5 and SP-7 are "not bad" but the tube units of that era are much more musical.
Of course, the SP-10 preamp is considered by many to be one of the best preamps ever built and the SP-11 was also a great preamp too.
Hope my input helps on this older posting.
