If your concern is network, get a Medicare Supplement, NOT an Advantage Plan. For Advantage, the feds give your Medicare premium to a private company like United Healthcare, and it operates like an HMO with a network and a huge interest in keeping the cost of your care below the "capitated" amount they are paid by CMS.
As far as the prescription costs, when you are looking at a Part D provider, check their formulary to see if it is covered and what your costs will be. Its a bizarre system, you can't count on a steady monthly cost because you go through different phases. You might pay a $250 copay in January because it includes your deductible and $50 in February then suddenly in June it goes back up and then goes way down again in Sept depending on what phase you are in and your overall (not individual drug) out of pocket costs are. Honestly, I haven't cracked that nut, and I use GoodRX for my pain meds because they arbitrarily cut them on me earlier in the year.