You can butt panels together as long as the doors open 90°
The absolute minimum distance between loadcenter cabinets, enclosed switches, or similar distribution enclosures is set by the 90° door-opening rule in NEC 110.26(A)(2):
(2) Width of Working Space. The width of the working space
in front of the electrical equipment shall be the width of the
equipment or 762 mm (30 in.), whichever is greater. In all
cases, the work space shall permit at least a 90 degree opening
of equipment doors or hinged panels.
However, you generally don't hit that save for corner cases; what will determine the actual minimum is loadcenter covers (combination covers tend to overhang their respective loadcenter boxes) and nipple sizing (you can't get any shorter than a close nipple) for surface mount installs, or the width of studs (1.5") for flush mount panels.
And yes, there is a maximum height for panels, measured straight down
The issue with the steep slope in your alternative location is twofold. First, a steep enough slope may simply not permit "ready and safe operation and maintenance" of the equipment in question, but that's an AHJ call. More importantly, though, the NEC sets a hard limit of 6'7" from the floor or ground level to the operating handle of the highest overcurrent device in the panel in NEC 240.24(A), as the exceptions to that rule don't apply to your situation. (This is so that you can get at your breakers in case you need to reset a tripped breaker or shut a faulty circuit off in a pinch, even if you aren't the Jolly Green Giant.)
Ditching the main breaker is actually a good idea
Since you were wondering, no, you won't need a main breaker in the subpanel provided that your subpanel is in or on the same building as the panel you're running the feeder from (and that you aren't invoking the feeder tap rules, but that's neither here nor there for you most likely). In fact, choosing a main lug panel here gives you an advantage, as many main lug panels ship with separate ground bars fitted at the factory, which saves you the work of having to fit them in the field. Just remember that neutrals go on neutral bars and grounds on grounding bars, in addition to pulling the green bonding screw or strap out of the new panel and running all four conductors (hot, hot, neutral, ground) to your new subpanel.
GO BIG OR GO HOME
Last but not least, there is no reason to pinch pennies on a tiny subpanel here. I would recommend a 40-space or 42-space, 200 or 225A, main lug panel in your situation as a minimum, since you're already starving for panel spaces. If you can, you might as well go bigger: the 2008 NEC allowed panels to have more than 42 spaces generally, so 54-space and 60-space panels have become available, if a bit pricier than their 42-space counterparts.
