I don't think the font and font size matter, because the final ebook (at least for kindles) will have a standard typeface. I write in Word (Times New Roman, 12 pt) and format in Sigil, then convert the epub to mobi in Calibre. Then I email the mobi file to my kindle fire to see how the final product will look. The default font of my fire is Georgia, but I can change it to Caecilia, Trebuchet, Verdana, Arial, Times New Roman, Courier, or Lucida. But if I were to use a different font when originally writing in Word (like Dolly, for example) it wouldn't show up as that font in the final product. It'd be the default Georgia. Somewhere along the way, the font gets standardized. So, like I said, don't get too hung up on font and size, 'cause in the end it won't really matter.
As far as formatting goes, you can certainly do it yourself. There's a bit of a learning curve, but I think it's well worth it to learn so you don't have the added expense of paying for a formatter with every book you release. Sigil is what I use, and it's free. The program I use to convert Sigil files (epub) to kindle files (mobi) is Calibre, and that's free, too. But if you really feel like getting a professional, there's plenty here on kindleboards. Try doing a search for "formatting" in the search bar on the top right of your screen. Hope this helped!
By all means you should strip out your font and size from the formatting when you convert to an ebook format. Most ereaders have their own specially chosen font and it's best to respect that to give the user a consistent experience. Many can't even display your font of choice even if you embed it, so it isn't worth embedding.
I write in Word and save as filtered HTML. Then I pull out any spurious span tags (Word is terrible about those), remove the @font-face style declarations, and in all my styles I remove the font family and replace it with something generic like serif or sans-serif. When I did my last book I did leave the sizes intact, but I think it's safest to remove them entirely and only use relative sizes (e.g., font-size: 120% for a chapter heading) as needed.
As others have said, definitely don't try to set the font and font size in your ebook. It will cause nothing but problems for your readers. If you're uploading a Word file, KDP should strip out the font things itself. If you're going with HTML, don't define any fonts or font sizes in your styles, and keep everything to plain text (with bold/italic/etc) or header tags.
There's a heap of information out there on formatting books. Guido Henkel has a great series of articles, starting at http://guidohenkel.com/?p=57
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