When a plan expires, you actually have up to 89 days after the expiration date to reactivate it before you lose the plan and phone number. So in your situation, since you want time to think about what you want, you're okay leaving it expired for now, without buying anything.
In any case, you can't just buy the $5 base plan and nothing else. In order for the account to be active, you must purchase at least one of the talk, text or data options. Another important point is, if you were to buy a cheap talk/text option just as a placeholder, that would cause you to lose any promotional or grandfathered plan you had previously. When you then buy another plan, you would only be able to select from those that PM currently offers.
Since your account is expired, PM will only allow you to reactivate it yourself if you renew the previous plan you had. Any change to a new plan on an expired account can only be done by a moderator. When you eventually decide on a plan you want, if it's not the previous one, you would need to contact a moderator by private message telling them the account phone number and the details of the plan you want. How to contact moderators:
http://productioncommunity.publicmobile.ca/t5/Knowledge-Base/Updated-Contacting-our-Community-Modera...
