Ask HN: Cloning basecamp

I have been using 37signals products for the last 2 years. On a recent project of my own, I built some basic project management using ideas from basecamp. It turned out that it was incredibly easy to build given the simplicity of basecamp (and the other 37signal products) as well as the ease of "just cloning the product without doing any thinking regarding features and user-interface design".

Now I am motivated to build a basecamp (and may be other 37s products) clone. I know the amount of effort that has gone into all the features and minor details of creating their products. I also don't want to claim that I can build a clone "over the weekend".

My intention is to build a clone with just enough features for my existing customers and market it, may be, just like 37signals. I am already profitable with other ventures so I am not in a hurry. Once the products are out, I can improve them and make them more feature rich with release-early-release often. I shall, of course, be charging less than 37s.

My questions to you guys are: 1. Is is legal to do this; copy features and design from an existing web application and building your own. If not, what can I do to remain legal. Please note I shall not be doing copy/paste from 37s, I shall be building everything from scratch.

2. Can I market my products with claims such as "it is basecamp but with less features" without offending 37signals?

I have been convinced by 37s that a product/service can still do a good job even with less features so I just want to do exactly this.

Edited title.