I owned a 37' -2000 model for several years. It is a fast and well behaved Bavaria, both as cruiser and and ocassional club racer, higher balast ratio and less flat bottom than later years Bavarias. Mine had an upgraded in-mast vertical battens mainsail of cruising laminate fabric ,anew from factory which served me well 6-7 years I owned the boat without problems .
I wouldn't touch the regular basic in-mast furling mainsail of that years models ,too weak ,unsupported, flutter in anything above 15 knots wind and lose it's shape fast. If not the upgraded in-mast sail, go for regular fully batten mainsail with single-line-reefing + lazy-pack combination, mostly supplied as standard by the German factory.