Another vote for the Marlin 1894. If you are familiar with guns at all, which I presume you are since you already own a GP 100 and handload, you will have no problems. Use what you already know about inspecting firearms. The 1894's are very robust and well made rifles. Not much goes wrong with them. However be leary of "Remlin" made 1894's. The Marlin rifles now made in Ilion, NY, have been wrought with quality control issues.
Here is an good article on what to look for in general.
http://www.chuckhawks.com/used_rifle.htm
Below is a good write up of things to look for on a used Marlin rifle by member tomray on Marlin Owners forum.
......What I'd look for in a used Marlin............
Condition of the Bore: ......., Ask for a bore light.....should be rust and pit free, but a few pits won't hurt accuracy in my opinion, in a hunting rifle.........Although pits in the bore at the muzzle can be troublesome........Check the crown for damage and dents too.........
Over all condition: ...........Has the rifle been mis-treated?........... Neglected?.........I'll buy a well used rifle, but not a mis-treated / neglected one............There are too many others out there to choose from......A little light rust can usually be removed with oil and 0000 steel wool when no pits are present.
Stocks: ...........Are they tight?....... do they match?............do they fit the metal well?.........Does the butt plate and grip cap fit right?.....Original finish?.......Check for evidence of cracks and/or repairs.
Bluing: ............This is purely subjective........"Beauty is in the eye of the beholder".....a well worn blue finish shows patina, and would not stop me from wanting to own the rifle.
Action: ......Is it smooth? .....or hitchy?..........Does the lever latch, and unlatch as it should?
Trigger: ..........Is it smooth?.........Have excessive Creep?..........Show signs for someone tinkering with it?............Is the trigger Original?......I wouldn't buy a Marlin rifle with an "after market" trigger..........And Yeah, I mean Happy trigger here........I'd toss it for original parts and then tune those parts.....but thats just me.....
Main Spring: ......Does it feel "FullPower"?............Or show signs of being shortened to reduce power, and as some think, "Enhance" the trigger?
Screws............Do the screw head slots show signs of being buggered up?.........Are all the filler screws present?....Any action screws missing?
Sights..............Are they both present and complete with front hood?..........Do they look original?...Are they both tight?
The Carrier:.........With the action closed, see that the carrier "tail" is free to move in the opening of the TGP. Open the action, and see that the carrier lifts to the loading position correctly....... NOW, watch the carrier as you slowly close the lever. The carrier should be supported by the follower in the high position until the bolt has moved about a 1/4 of its travel. If this is not the case, ther rifle likely has "Last shot loading problems, and needs attention in the carrier/follower area.
You will likely find small issues with all used hunting rifles........Only you can decide if they are O.K as is, or will need correction, and can you correct them easily for little or no additional $$."