So, how do these modern-day hired guns get by?
A bigger question might be how errant mechs get created in the first place. Armies are usually loathe to allow a major piece of military equipment to walk off (or be secreted out of) a base, nevertheless launched offworld and jumped into the unknown Periphery. And it's hard to see how a mechwarrior who owns his ride and is rich enough to afford interstellar shipping to a very out-of-the-way world would choose to spend his riches on the errant lifestyle in retirement. Even if seeking solace from PTSD, setting oneself up to take on annual bandit raids singlehandedly seems like a bad way to deal with the psychological scars of warfare.
I think it's more likely that errants are forced into that lifestyle rather than choosing it. Mechwarriors who are left behind on a world due to retreat or accident and with no other way to make a living seems like a more probable source of errants. But even this will be a rare event.
So aside from a very few eccentric and rich and a few very desperate mechwarriors, it's hard to see how errants come to exist in the first place.
But these are problems throughout the btech universe. It's also hard to explain many mercenary, family, and gladiatorial mechs. So handwaving the origins questions away and going with the romance of the errant mechwarrior for a moment...
I'd assume that without the backing of a military logistical train, that an errant mech will run out of usable ammo in a matter of years. Even if the errant mechwarrior brings a large store of ammunition with him, high-grade explosives tend to degrade with time and his ammo will become dangerous to handle or turn into dummy rounds (or both). There are no rules for this, but unless chemistry works differently in the btech universe, it's unavoidable. This may also be true for other consumables like coolants and lubricants, depending on their composition.
The exception to this would be if the local militia uses some of the same heavy weapons and equipment on their combat vehicles, field guns, or emplacements. If the militia has vehicles, autocannons, missile racks, and heat sinks, then the errant may be able to leverage their supply of lubricants, ammo, coolants, etc.
If the militia doesn't have the same heavy weapons, then it may also be possible to modify the errant mech to use what the militia does field. Specifically trading RLs (maybe MRMs) for LRMs and SRMs and maybe rifles for autocannons. Hopefully there's enough heavy machinery requiring the right lubricants regardless, but the absence of heat sinks on-world and a source of their coolants might pose a limiting problem.
Even if they can leverage militia ammo and consumables (or industrial consumables), I'd also assume that errant mechs can't get by for very long before suffering the kind of permanent damage that showed up in the TO&Es in the old Succession Wars-era scenario packs. Things like a non-functional heat sink, a stuck actuator, sensor impairment, an improperly aiming weapon, and structural failure that reduces armor capacity (even when repaired) in a particular section of the mech. Even if the errant mech sees no combat, it will be a matter of years before certain components to wear out from fatigue or the environment. Seals will fail, optics will go out realignment, electronics will short out, fasteners will oxidize, reactor structures will suffer neutron bombardment, etc. When they do, some malfunctions may be resolved with jury-rigging but others will be irreparable without replacement components and the right tools to install them.
So to sum up, to realistically portray an errant mech, I'd say it has to be able to tap into militia supplies or at least supplies for local heavy machinery. An errant will not last long protecting an Amish village. An errant mech may also have some modifications to accommodate the weaponry and ammo that is available locally. And an errant mech is probably also suffering from one or more permanently malfunctioning components.
At least, that's what I would do if I was GMing a campaign with an errant. Anything less is either a newbie errant or unrealistic, IMO.
What kind of 'Mechs are best suited for the job, and how much could your average world do to keep them operating?
As others have already stated, assuming the errant has a choice, he'll want a relatively well armored and mobile generalist that relies mostly on energy weapons to reduce the ammo problem. But realistically, an errant is not going to last long without some militia supply chain or semi-modern industrial base to leverage, and even then, the errant mech may require modifications and will probably suffer long-term malfunctions.
My 2 C-bills, FWIW.
