Personally I find the plastic reels for the 2500 tanks very easy to load.
You need to ask yourself some questions.
1) Do you want to use the 2500 or the expert tanks. Those are the only choices for 4x5 and bigger. If you need expert tanks make sure the machine you're looking at will handle them.
2) Do you want to run the 120 and the 4x5 in the same run? Then your only choice is the 2500 tanks. You'll need a tank big enough to handle at least one 2509N reel and one of the roll film reels. I guess that means the 2551 or 2553 tank or bigger. The more film you need to run the bigger the tank. That will rule out the smallest machines. The above setup [one rollfilm reel and one sheet film reel] will handle 2x120 and 6 sheets of 4x5 all in the 2551 type tank. Going to the 2581/2583 will add I think two more rolls of 120 or maybe 6 more sheets of 4x5. The last number in the tank is the type of lid. 2551 is a normal lid for use without the lift. 2553 is a cog lid for use with a lift. That's what the 1 or 3 at the end mean. The lids can be swapped. The normal lid can be upgraded to the cog lid but you can't go back the other way.
Check what comes with the machines. Often used machines are sold with a complete set of tanks. Odds are you'll want a range of tanks from the very smallest to the largest. Used tanks are pretty reasonable. Using a smallest tank with colour will save you chemicals usually.
You might want to surf over to the Kodak website and read all the info they have on E-6 processing. You'll find pages and pages of stuff.