an 84 fiero is an ok car... if you're paying like 600 bucks, especially if its a 4 banger. the 2.5L GM I4 was only 88hp, and it was a heavy mother for an I4, but 84 fieros in general are pretty crappy and not worth alot.
what you want is an 86 fiero, the 85 is ok but the 86 is ideal, be it in GT form with the 2.8L V6, or the lowly 2M4 2.5L I4 its a good car, the fiero is small and light so the I4 is decent for it.
the pontiac fiero has a steel spaceframe and underbody with plastic bodypanels, including the doors, and the roof, everything is plastic, except for the steel floor, and steel frame.
the 2.8L V6 made 140 HP and 165 lbft of torque, i believe, or maybe it was 135hp, who knows. its plenty for such a small car.
the 2.5L I4 made 88hp and probably about 120ish lbft of torque.
the I4 is adequate, but don't be willing to spend alot of money on a 4 banger fiero... only get one if you don't plan on racing it in any way, or you plan to do an engine swap. not that its a bad engine, its just not very powerful.
the 2.8L V6 was used in celebritys, GMC Jimmys, cavaliers, chevy S10s, and alot of other GM cars and trucks in the 80s, i believe the fiero had a different exhaust and intake manifold or something, its 5 hp more than the celebrity version @ either 140 or 135 hp.
the fiero 2M4 and 2M6 share the same body (the 2M4 is the I4 and the 2M6 is the V6). from in 1984 i believe there was only one nose, and i can't remember about the SE and GT models. in 1985 the fiero GT had the same body as the 2Ms with a different nose with an air dam, the GT only came with the V6. the SE i believe also had the GT nose, but i cant remember if there was a 2M4 SE or whatever, i doubt it. in 86 it was the same deal. in 87 the Fiero GT had the same GT nose as before but with a new rear body, the other 2Ms and SE remained the same, i believe the SE had the GT nose. in 88 (the last year of the fiero) the car got a completely redone suspension (previously the front end steering equipment was from the chevette and the rear transaxle stuff was from the front wheel drive chevy citation, like a toyota MR2 they simply moved the FWD drivetrain to the back 'o the car.) which gave the car much better handling. the 2M4, and the Formula (which i believe is the 2M6) kept basically the same body, but the lines were rounder. the SE, if it was still there, had the GT nose still... and the GT had the whole GT body like in 87... i can't remember if the 86 GT had the whole GT body, maybe it did.
anyway, the 88 fiero GT is the best, and i believe the rarest fiero is the 88 GT with T-Tops, as 88 was the only year for fiero T Tops, iirc.
enjoy fieros!