It's a bit like asking which car to buy.
Most new computers will easily handle the Microsoft office side of things, but for the AutoCad and image files you will need a reasonable amount of RAM. Go for something like this, and you should be fine.
Cloud services are many and varied, but they all work in the same way - you upload your files to a remote server, and access them from anywhere in the world. Security is of prime importance where business users are concerned, and if you're going to store files in the cloud make sure you have a separate backup on some kind of removable media. In the field this can be a USB flash drive.