If you need 100% compatibility for documents, buy Microsoft Office.
If you want to buy a security suite, I suggest you give a look at ESET Smart Security which is as good as Kaspersky or BitDefender, but lighter. Norton is also a good alternative -- I still have to figure out why so many people do not suggest it.
If you want to go free, Avira and Comodo Firewall is a good combination.
CCleaner, Malwarebytes, IObit Malware Fighter and Advanced SystemCare are a must-have for me. I also use both Defraggler and Smart Defrag 2. Notice I use two softwares for the same things, that's just me. It's all down to you.
For email, Thunderbird is awesome, it even has extensions that integrate a calendar if you need one. If you buy Microsoft Office, stick to Outlook -- I am loving the last version but I still use Thunderbird sometimes.