Basic things should be finished first. Functionality extensions can be added later on.

Let me give an example. This forum Nameslot.com was running on phpBB. Then I changed it to MyBB. But lost some indexed pages and all that stuff. I knew this scenarios but then at that time I didn't even had money to buy in the forum script.

I uploaded the script and made the categories. Also installed the SEO friendly URL's mod at the very beginning so not to lose the indexed pages by moving from here and there.

Added some subcategories later on and then added the cash mod and the status image instead of "Online - Offline" text which was just spamming the index of search engines and decreasing the value of the page.

I don't know what kind of project you are planning but if you can let us know in Help wanted section then we can discuss it out.

Thanks.