Given the emphasis on not being detected, particularly against Cappies, go high-tech with an Angel ECM and vehicular stealth armor from the RAF’s Bolla variant.
Armor should be thick enough to withstand one PPC (minimum) or Gauss (goal) snipe on any glacis. This includes the rear because this unit should be in full retreat if it’s being shot at. (Alternately allow the Power Reverse design quirk.)
Since this is intended to scout mech forces, it needs to be able to go (most of) where they go. On the ground, that means a tracked vehicle, ideally with some degree of amphibious capability.
Fusion power plant for long-range, independent operations. You want to be able to outrun nearly all mechs, so a minimum speed of 7/11 and ideally 9/14 or higher. I’m imagining something like the Ripsaw UGV. Supercharger for really fast retreats would be a nice option.
If it’s really not intended to engage enemy forces — a true look-and-do-not-touch recon unit — I’d forgo the active probes and TAG because it should never get close enough. Instead, mount a Recon Camera, Sensor Dispenser, and enough Communications Equipment to run the sensors and report back.
In case stuff hits the fan, I’d include some Vehicular Grenade Launchers with smoke ammo to cover a retreat. Alternately, if there’s enough space, some MML tubes with enough ammo tonnage for a couple to a few ammo types, including smoke. Probably need a machine gun for hosing down any enemy infantry patrols this unit runs across.
Other protective measures like an armored motive system, anti-missile system, or mine dispensers/FASCAM ammo (to cover a retreat) would be nice-to-haves but not baseline requirements. Same goes for crew amenities that don’t apply in a BT game but add flavor given the long-duration nature of their missions.
The suggestion upthread about carrying an infantry squad makes real-world and roleplaying sense but may not have much benefit in a BT game.
Someone else mentioned the Boomerang upthread. There may be a VTOL or conventional fighter version of this with most of the same kit that does the job persistently at altitude.
If low-tech and Angel ECM/vehicular stealth armor is not in the budget, then buying lots of the aforementioned Boomerangs, jeeps, skimmers, and the like probably makes the most sense. A modern version of the Rotunda might be worth the investment to avoid suspicion (but not detection).