What secondary specialization should I add to my bounty hunter operator? I am thinking of rigger or modder? I want to repair and modify my ship and gear.

I want to buy a Lancer-Class Pursuit Craft. Is this ship any good? I know that it has three encumberance points. It needs a reinforced shields for the aft. I know that I want something for stealth to add to the ship. I want to add encumberance points to this ship but the rigger talent options are great too. Or should I buy a VCX-100 Light Freighter and modify that?

Can a Cathar be trained as a mandalorian bounty hunter?

Modder is the only way to add HP to vehicles, while Rigger offers significantly more Signature Vehicle upgrades. It really depends on how much those HP mean to you. Modder is split between modifying ships and modifying gear. If you want to modify gear, I'd go Gadgeteer, which saves some XP and also allows the modification of gear, and then pick up Rigger, but YMMV. If you want to dominate in a cockpit, Pilot is a good thing to enhance Operator with. Master Pilot and Improved Full Throttle are indispensable talents for pilots, as well as Skilled Jockey.

Two things about the species: Cathar aren't officially in the game, so anything you're using is (obviously) home-brewed, and the Mandalorians aren't statted to include other species. As a GM, I would say that if you wanted to to have a Mandalorian Cather, that's acceptable, but there would be no outside changes to any species abilities--it's all backstory.

Anybody can add attachments and modify them. Rigger and Modder have some additional stuff but honestly I'd just stay in BH and go Gadgeteer. It has Jury Rigged and Tinkerer, will let you stay in BH, and is frikkin great.

I personally think the Lancer is crap. Mod a YT-2400 and you'll have a better ship by far.