The Hector and the Marauder 4X can proxy well too with some kitbashing.
I'm not a huge fan of the Hector, when there are better options...
The MAD-4X Marauder (20-5085) is actually my second choice, but one that requires entirely too much modification to make a good looking Unseen stand in.
(Note: The arms and dorsal turret are parts that would be hard to find, so bashing an exact copy is not recommended)
Seconding the Marauder IIC as a start, but it fails the basic MAD test for me - the arms aren't in front of the legs.
Obviously I feel the same way... But I am looking for the best option, that doesn't require any modding.
And welcome to the club. Once you kitbash, there's no going back ...
This is also true... I seldom paint anything that hasn't been bashed/reposed in some fashion anymore.
The thing is, modding is a skill set that actually encompasses skills like; cutting, pinning and sculpting. And, these skills all require their own tools (yes, makeshift ones can be used, but there does come a point that one does need to invest).
I would take the entry into modding in steps. Start by just learning to pin; you'll need a pin vise, some type of pinning material (I prefer .020 music wire) and the appropriate drill bit (for .020 you use a #76). Multiple piece 'Mechs are a good place to practice. Then maybe add in a little sculpting (green stuff/Kneedatite is commonly used). Use this to clean up your joints and fill in spots where you pinned parts together. If you can get a result you are happy with, then maybe next time cut up/off an arm or a leg and repose it... I would do all of this over a few Mechs, building on each to give you confidence to do the next.
As Worktroll has said, "It looks potentially intimidating, but is not as hard as it might seem" (granted, I am taking him slightly out of context)...
Modding is the gateway to a unique collection, and the answer to "I can't find what I am looking for", but until you are ready, it is something that can wait.
Caz