That 564B Scope is a wonderful buy. I own one of these along with quite a number of Tektronix 7000 series scopes, and my 564B get regular use as the Output Display system, on my EAI TR-20 Analog Computer. Not only is it reliable but in the winter months give my workshop a nice warm feeling.
Ok the TDR modules may not be suitable for general purpose work, but I see you have an offer to exchange them for a Amp and Time base module. Do your homework on the various Tek Amps and Timebases that are available for this series if Tek scopes and ask Martin what are model numbers of what he is offering. Then you can get a feel if you will find this a good deal.
The valves in the 564B are not that difficult to get replacements for, and service manual is available on line, either as a pdf (
http://bama.edebris.com/manuals/tek/564/) or to buy a genuine hard copy. Personally I would recommend you check out
http://www.etestmanuals.com/ for a deal on a genuine manual for your instrument and plugin's
While it would be nice to get a more modern scope with TV Line and Frame sync triggering, its not that difficult to rig up such an external sync separator unit; and feed its output into the ext trig input of most scope timebases.
I just went and looked at the modules I have installed in my 564B, one has valves and the other is solid state.
Owning quite an assortment of plugins, I at times use the 564b with fully valve based plugings and other times both are fully solid state one. So you should not have an issue with your 564B running any of the family modules.
download the Tektronix Catalog for 1963 and look at the info on the 564 and the modules
http://bitsavers.trailing-edge.com/pdf/tektronix/catalog/Tektronix_Catalog_1963_%2322.pdf you may choose also to look for a later circa 1968 or earlier at
http://bitsavers.trailing-edge.com/pdf/tektronix/catalog/regards
David