We generally don't use tables for buttons. Instead we use the following for max-clickable area, border radius and MSO support. It makes for a nice looking + functional button.
<!--[if mso]> <v:roundrect xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="urn:schemas-microsoft-com:office:word" href="XXX" style="height:42px;v-text-anchor:middle;width:200px;" arcsize="40%" strokecolor="#D4D4D4" fillcolor="#033C5A"> <w:anchorlock/> <center style="color:#ffffff;font-family:sans-serif;font-size:16px;font-weight:400;">BUTTON TEXT →</center> </v:roundrect> <![endif]--> <a href="XXX" style="background-color:#033C5A;border:2px solid #D4D4D4;border-radius:20px;color:#ffffff;display:inline-block;font-family:sans-serif;font-size:16px;font-weight:400;line-height:42px;text-align:center;text-decoration:none;width:200px;-webkit-text-size-adjust:none;mso-hide:all;">BUTTON TEXT →</a>
Always build. But there is tons of framework to build around out there. No need to start from scratch it you know why you need. But if you want what you want in your mind, start from scratch. Never failed me. If you have a hiccup, post to the discussion or seek out advice on Twitter. Like I said, I've never gone wrong with this methodology. Trying to work with a template is always more difficult, in my opinion.
Litmus wrote about this not long ago... Silly smart watches are making text versions a must: https://litmus.com/blog/apple-watch-favors-text-version-breaks-links
Helvetica, Arial, sans-serif to be safe. I do like to use open-sans, but there are still those handful of pesky ESPs that don't support web fonts.
This is an awesome resource: http://www.cssfontstack.com. It gives some great web safe font-family combinations and breaks down percentages of rendering by OS.