If the truck was garaged; how did lightning get in the garage to hit the antenna? Seriously; if the power ran down the antenna; it probably fried the stereo or whatever entertainment system there is. But the unknown factor is what it did beyond that. I would look as thoroughly as possible under the dashboard at the fuse block, the wiring harnesses, the steering column wiring, and try to see the back of the instrument panel and the heating/ventilation controls. Look and sniff for burned wiring and for discolored wiring plugs. Also look for burned wiring under the hood. If there is a shorted harness in the passenger compartment; and you replace a blown fusible link; the new link will probably fry immediately. So; after first disconnecting the heavy power wire at the alternator, and covering the metal connector on the end of that wire with tape or wrapping it in a rag; it might be worth initially using a length of 14 gauge wire to bridge across the blown fusible link, and seeing whether there is a big spark when the wire first makes contact; but do not leave the wire connected for more than a brief tap. If there is a big spark; there probably is a short somewhere. It could be in the door wiring, or the wiring to the overhead lights, or the brake light or headlight wiring, or anywhere in the electrical system. If you disconnect the harness plugs one by one; and keep checking for a power draw through the fusible link circuit after each time; when the draw stops, the last plug you disconnected would lead to the shorted section.
If you clear the short; it then might be worth briefly connecting the alternator power cable again (once there is a charged battery in the system), and see if there is a spark when the cable touches the alternator terminal. If no spark; the alternator might be OK; but I would be surprised if this was the case. The battery may or may not be still chargeable. The headlights sometimes can be used as a tool to see whether the battery has power in it; because the headlights are independent of the ignition switch; while virtually everything else goes through the ignition switch. Ignition switches can fry internally; as can any other kind of switch or relay.
If there turns out to be a big short somewhere; then you'll have to decide whether it is worth buying the truck without knowing how much work will be required to fix the electrical system. If the wiring has to be ripped out of the vehicle interior; it may turn out to be a real pain to do. That would also depend on how skilled you are with electrical work; how much time you have, and what kind of budget you'll have for the repairs. This might get too difficult for your knowledge of electronics. If so; it might be worth finding a local person who can do electrical troubleshooting and pay them to check the system out. Or; if it seems too much to take on; it sometimes is wise to know when to quit. There's always going to be some opportunity out there which requires more than you have to do it right. Regardless of how nice it is; some projects are just too much to take on.