2. Is there a ban on donations from foreign interests to candidates?

While there is no ceiling on private contributions from individuals or legal entities, most parties met with by the OSCE/ODIHR NAM stated that they will rely mostly on public funds.
Source: OSCE/ODIHR (2016), Republic of San Marino, Early Parliamentary Elections, 20 November 2016, OSCE/ODIHR Needs Assessment Mission Report, OSCE/ODIHR, Warsaw

There is no ceiling on private contributions to political parties, or on how much the latter can raise from private sources.
Source: GRECO (2016), Third Evaluation Round, Evaluation Report on San Marino, Transparency of Party Funding, (Theme II), GRECO, Strasbourg