For my contract template, I cobbled together relevant parts of several sample contracts I found online at...oh geez...I think the site had a very obvious name like freelegalforms.com or something similar. You'll find that there are sections of the basic "contractor" form that don't apply to you (they are written for building contractors), so some editing will be necessary.
You are welcome to borrow from my contracts if you like. I have a samples posted on my Web site, linked from the Rates and Services page. I don't think I can post my Web address here, but you'll find me easily enough if you Google my company name.
That said, once you have a basic contract/letter of agreement/any legal form together, it is always advisable to run it by a lawyer with experience in contract law in your state, just to make sure.
On the invoicing question, many VAs simply set up a detailed internal spreadsheet to keep records, then do a selective merge to prepare invoices for clients. This gives you plenty of flexibility and options for customization. I started with that, but currently I'm using a great little open source (FREE!!!) program that works just great for a very small business: Volutive. It takes a little experimenting to "get" how it works, but once you get the feel for this set-up, it's very easy to use. Check it out here