Navigation is relatively easy when it comes to open-source ecommerce software. (For instance, Brumby, take a look at
www.soundssublimeshop.com. You can see that the various categories are easy to search through, by selecting from the drop-down menu.)
There's also a database-based search function to help with finding the specific product if the buyer doesn't want to browse.
The individual look comes from editing the CSS and of course altering various of the php pages to change the layout.
Quite a bit of editing comes into play if you're thinking of using oscommerce for a bookshop, for instance - because by default there is no field for author, format, ISBN, etc. You would have no problems, I'm sure, in dealing with the editing involved, and if you need to increase the maximum width for some of the fields, you'd be able to edit the database in MySQL without having a breakdown (as some newcomers do! "What's MySQL? Which file is it? Help!").
Interspire is outrageously expensive - if you plan to have more than 100 products, it's $995 for the licence (and one year's maintenance).
As you know, anything open-source is free.
I very strongly believe in a consistent LOOK for a website - something that matches the business cards, the letterhead, any signage or posters, any publicity material, etc. This is part of what makes a site memorable - the colour scheme, the logo, the fonts used, the arrangement.
It is beyond question of prime importance to load the main page quickly, and also to have good meta data. It helps a great deal if the main page includes keyword-rich text, so as to reinforce the value of the keywords and descriptions in the meta data. There are other helpful ideas to get strong page ranking, such as using clean code (Google will actually rank pages with cleaner and better code HIGHER than pages with junk-filled poorly-written code), including alt-text for all images, having no dead links, being xhtml-compliant and using valid css, using keywords as links to keyword-rich other text, etc.