Definitely use a mojoPortal skin to start off mainly because there are a lot different features in mojo and you should define a style for these in your skin. With your PSD, the best way I have found is do exactly as you propose and slice it up in PS, make it all work with XHTML/CSS and then add this style to the mojoPortal skin you chose. You will have to merge some of your selectors into existing mojoPortal selectors such as .center-nomargins, .center-leftmargin, etc...
And as always, feel free to ask specific questions here on the forum! We will always try to help.
-Joe D