Go with the Tak 55/2, unless you can bag a Meyer-Optik Oreston 50/1.8 for a Tak 55/2 price. I LOVE my Oreston. Close focusing, incredible bokeh, scalpel sharp.
I had a Helios 44-2 preset that I used on my SP1000. worked like a top, but I sold it to an acquaintance because I did not like having to remember to stop down before the shot. I understand the later 44-2 lenses with auto stopdown pin work just fine on Spotties.
You can ask that question at
www.mflenses.com. Super manual focus lens site. Tell 'em fish sent you ...