used car parts, from england to usa? duties? customs? currency conversions?

I'm considering buying two (specialty) =used= car carburetors from
england; the deal's 'all agreed to' but awaits my payment. meanwhile I
have no CLUE where to investigate wether or not 'duty' or 'customs'
applies to the transaction (google searches are no help, either), and,
if duty/customs DOES apply, who pays it at what end, when?

I have a paypal account, and I've bought stuff over ebay before (this is
not an ebay transaction, however). I understand paypal will do the curr
conversion at some (what I read is 'exhorbitant') rate, and/or that
possibly mastercard will also do it for me (at some lesser percent
additional???) Are there other options?

I also have no CLUE how it would work with currency conversion (I mean
the 'ratio of conversion' part is straightforward enough, but who pays
the additional 'percent' for converting my dollars to UK pounds, where,
and when, ordinarily?

and, what'd be the cheapest way for seller to send ("mail") two used
carburetors to me? time is "NOT of the essence" for my needs...by boat?
would that normally take a month or two? more? then they "get tied up in
customs" somewhere along the line? or possibly get mistaken for bombs by
the x-ray inspectors?

thanks for tips and education,

tool box