A B Rank would be pushing it but that's probably her limit. I could see her struggling against an A and an S would be pretty unlikely, to say the least. But I'd say that puts her on par with Kuwabara? I think Genkai mentions the general rank of everyone in the group in a later episode during the Chapter Black Saga...I can see Botan bringing up training, or if Kurama does, she would agree and consider what she could do. I see her being more into healing than fighting but the tricks she learns would be pretty awesome, without a doubt. Botan might have to forever wear roses in her hair (Kurama's way of staking his claim in a classy way XD). But yeah, she's fucking hot, a grim reaper, can heal and is dating Yoko Kurama? Damn girl!

Kurama and Botan can go as a doctor/nurse combo. There's so much to play with. But hey, then you got the school parties with their games and drinking and all that jazz. I have a feeling they'll be flooded with questions throughout the first month they're together, and I think they have 'year round' schooling so they can have summer swim classes and everything else~

As long as Botan doesn't say "See you soon!" to Shiori when they're talking about her reaper duties. I'm sure Botan would quickly correct that she didn't mean she was dying anytime soon, so many misunderstandings. But I think it would make the bond between Shiori and Botan even greater that they both share the secret of knowing who Kurama really is. Plus it's good his mother and her get along since Botan seems pretty genuine and to let Kurama relax a little and enjoy life more. But yeah, we can have plenty of demon scum be stupid and sign their own death warrants.