According to the new HDB rules, you can only sign the seller checklist after MOP. Thereafter, you have to wait for the 7 days cooling off period before you can grant any option.
Therefore i make more sense to get the valuation report together when you sign the seller checklist.
You can get valuation done and buy buyer before MOP, but it will not speed up the processing time.
You can probably get an agent to get the paperwork done about 2 weeks before your MOP.
Hope I had answered all your questions.
Feel free to contact should you need any further assistance.
Thanks and Regards
Goh Mui Huat (Johnny)
CEA Regn No. R018297H
Dennis Wee Realty Pte Ltd