Q: buying ipad without cellular, would i be able to insert a sim card any time later?

Cellular can not be added to wifi only iPads.

The only way to connect a WiFi-only iPad to a cell network is via a MiFi device or by tethering through a cellphone. As Bob said, there is no way to add cellular capabilties internally to a WiFi-only iPad.

Regards.

You need to buy a Wifi + Cellular iPad to be able to use a sim card.  You do not need to get a cellular service plan at the time of purchase though.

But if you are thinking of wanting cellular service later you need an iPad with Cellular capabilites.

A Wifi only iPad can never be directly connected to a cellular network.

iOS: Understanding Personal Hotspot

http://support.apple.com/kb/HT4517

Use Bluetooth to tether your iPhone, iPod touch, or iPad

http://www.macworld.com/article/1159258/bluetooth_tethering.html

How to Connect an iPad to an iPhone Via Bluetooth Tethering

http://techtips.salon.com/connect-ipad-iphone-via-bluetooth-tethering-25472.html

Get a Mi-Fi http://en.wikipedia.org/wiki/MiFi

 Cheers, Tom