I am aware that a couple who are both Singapore Permanent Residents are eligible to buy a resale HDB flat in Singapore. Is this the only available option for an SPR couple, or are they eligible to buy other types of flats?
According to HDB, the eligibility criteria for a new flat in respect to citizenship is:
You must be a Singapore Citizen
This holds true for all the categories, you can check this by clicking through the categories on the left hand menu on the website.
Note that even for resale flats there is a restriction:
From 27 August 2013, SPR household (i.e. a household with no SC owner) will have to wait 3 years from the date of obtaining SPR status before they can buy a resale flat
This might be a concern for you if you only plan to acquire PR status.