“It would be much appreciated”: That is the passive voice; the agent is omitted. The active form would be “Someone / I / people / etc. would appreciate it”. The adverb “much” is sometimes used to intensify the verb phrase, usually with "not" or another adverb (like "very") before it. It can also be used to intensify a past / passive participle. “Much grateful” does not work in English, because we cannot use “much” as an adverb to intensify an adjective, but only to intensify a verb (and then, usually with a negative verb). The equivalent of “much” for an adjective is “very”. (“Much” can also be used as an adjective.) “Most grateful” is correct; it means as grateful as possible, or extremely grateful.