Looks like after I get back from vacation next week I'll be lurking Ebay.......so all you HP owners...raise your prices now.
bd is right about the 5334b. it does a lot and they are often found for peanuts. adding the c channel is a piece of cake. you may have to hunt afterward for a high stability oscillator if not already equipped.
the 5335a is a sexy piece of gear and does display more digits (with interpolation). you can't beat it for the number of digits it will resolve per second. but the fan is loud and because of its size shipping will be more.
a 5300b system with a 5303b will go to 512Mhz in a small form factor. but there is no internal high stability oscillator option. the 5300a system has beautiful dot leds......but only 6 digits (the 7 segment 5300b has 8 ).
BUT there is nothing wrong with the 5328a with a 512 Mhz c option. they are plentiful, often cheap, and reliable as gravity. they usually have a high stability oscillator (unless someone stole it). had 4 of them at one time. two reasons they did not stick around.....
1. they are BORING.
2. they are not nixie.
EDIT unless you are looking for a real challenge avoid the 5345. it is an amazing device....counts to 500 Mhz without prescaling. BUT they weigh a ton, you need at least 3 chassis to keep one working (had 3 and a box of spare boards), and the really cool plug ins are expensive as shit. they develop intermittent plated thru vias on the mother board and are chock full of aging gold ecl chips that run on magic fairy dust.