Using today's junk gas with a Magnum/LA is no issue. Biggest problem with ethanol in modern gas is the fuel lines (which, if you are rebuiiding a F, M, or J body anyway, should probably be replaced, so replace them with fuel injection hose that can stand up to ethanol), and the floats/seats in the carbs. You can buy brass floats, and clean the seats of any deposits, or go with a Street Demon carb, which is sort of the modern reincarnation of the Thermoquad.
There are easily more parts, and far cheaper for the LA/Magnums than for the Hemi. A air filter for an LA is about $4-5, Hemi is close to $20. Same with oil filters, athough the price difference is lower. LA/Magnums can use standard copper spark plugs, not sure that the Hemi uses copper, but I would bet that they are platinum or double platinum. Most other regular maintenance items are the same cost, but being cast iron, the Magnum/LA blocks are not as picky about coolant as the aluminum headed Hemi. Again, speaking from ignorance here, but Magnums/LAs get along fine with conventional engine oil, Hemis may require synthetic (but not sure about that).
Modern machinics would probably freak out if you brought an F, M, or J to them. First thing they would start looking for is an ODB II connector. If you have brought the whole drivetrain and wiring harness over, they may be able to work on it. More likely though, and especially if you have not used all (and I do mean ALL) of the factory original parts, they will be lost, or just not even look at it. Also, if the intent of this hobby is to be held to, you shouldn't be bringing your car over to "modern mechanics". If you want a factory spec. modern car, buy a factory spec. modern car. Long term, it will cost less than taking a drivetrain/wiring harness out of a newer car, working out how to get it to work in our F, M, or J Bodies, and you will be able to bring it to a "modern mechanic".