Opinions on Tarifa as a base

Take a jacket which will protect you from the wind

It's a great kite surfing and windsurfing spot. The town itself is a bit scruffy, but fun. I think it might not be as dead as you would expect in January because of the windsurfers. You can go to Tanger for a day trip - it is a bit of a hassle, but since you will already be close to the ferry port it could be fun.
The ruins of Baelo Claudia, a bit to the north, are really cool, and next to a beautiful beach with large dunes.
A bit further still, Vejer de la Frontera is a lovely white-washed hill town.
I would want a rental car in the area.

I can’t imagine spending two weeks there, but I have no idea what you want to do during that two weeks. If your lodging will be free, go for it. I’m sure whoever invited you will either have a vehicle or know the bus schedules and where the busses go.

I agree with Balso. If you wanted to windsurf or just watch windsurfers, nearby Punta Paloma is a great place. It has a nice beach you could walk along, out to the point.

thanks for the responses...January in Spain does not do too much for me but I needed to ask.