if it's a 77, it has the single Zenith-Stromberg Carb, 1-piece intake header, and 1-piece exhaust header like my 1979 had. my first suggestion is to trash everything that sucks air into the engine or spits it out. The 1975+ Z-S setup was lousy. Poor throttle response, poor quality, less power, and the manifolds were more prone to cracking.
i would highly suggest a dual-SU carb conversion or single Webber downdraft carb.
once you've updated the carb, upgrade the electricals a bit. you're running 12v in that car. increase the capacity by upping the alternator to at least 65 Amps. i think the factory unit was only good for 35-40 Amps. then upgrade the ignition, get new wires and plugs, clean the distributor and points, and check the timing.
once you have all that, you'll have a decent running ~80rwhp car that will be able to get out of its own way.
as for rust: the sills, doglegs, and upper front fenders are very prone. Check these areas thoroughly. The sills are especially important since they hold the car together on the convertible. these elements and the transmission tunnel are all that keep the engine from finding its way into the trunk! you'll have an MGB sandwich if you're not careful. And sill replacement is NOT as easy as it sounds. it is daunting because it has to look good and be strong.
finally, engine cooling: with a 77, i think you have a single electric fan for cooling. my 79 had 2 fans. my 74 had one mechanically operated by the crank pulley. whatever you have, get a bigger one and think about upgrading to a larger radiator core. MGB engines run hot. a big unit up front will keep the temps normal.
i'd try to pick the car up for $1200-$1300 considering you don't know the extent of the rust on MGBs until you look under the rug. And budget $3000 to fix it in case you need to replace the top and/or do any rust repair. the good thing is that well sorted MGBs are moving up in price. i've seen #3 cars on eBay sell for $5k-$6k depending on fit and finish.
and lastly, i have a box of spare MGB parts from my 79 that i am willing to send you for $10+shipping. PM me if interested. lots of extra nuts, bolts, hoses, brackets, etc...
