Nice to know about your Nepal visit in May.
1day Bhaktapur tour may not be enough because Bhaktapur is a district which comprises beautiful places like Nagarkot etc but as you have only one day remains so there is no other option.
You can get many buses(in every 15-20 minutes) to Bhaktapur from baghbazar near ratnapark kathmandu. These are local buses and sometimes a bit crowdy but much cheaper(NPR 20pp).
However, you can hire a taxi that is much much more times costly (NPR 800-1000). For one day tour I don't think it is wise to use any tour company.
You can get many freelancer guides at Bhaktapur durbar square( some of them are teenagers too) who can tell you all about D.S.
Doing a tour yourself is also an experience, Getting everything done sometimes doesn't give such experience so I suggest you to do it yourself rather seeking for any tour compay rest is your decision.