I like your time allocation, but I'm on the record as having been disappointed in Marseilles. I made the mistake of staying near the main RR station, which isn't a particularly attractive area. I did enjoy my walks around the port.
I think you'll be able to find a very reasonably priced hotel room in Nice during your time period. I had a long talk with the manager of the budget hotel I stayed in, during which he explained how hard it was to provide the quality experience his French clientele expected (they apparently insist on very nice towels) while still keeping his rates down. I gathered that occupancy levels may not be super-high outside of mid-summer and periods when there's a major event on the Riviera like the Monte Carlo Grand Prix, so rates can be quite soft.
I suggest looking at the website of the Fondation Maeght in St.-Paul-de-Vence to see whether it would appeal to you. I loved it, but art is a matter of taste. As much as the mobs of tourists in SPdV bugged me, I can't recommend skipping the place since you'll have a car at your disposal. It really is a beautiful town; it's just that every building seems to house a tourist shop or restaurant (rather like Taormina).
I really liked the Naïve Art Museum and Asian Art Museum in Nice. Both are out toward the airport but not particularly close together; I took buses on separate days (and got majorly hung up in traffic as I headed back into town on one of those days). I was told the garden near the Asian Art Museum is nice but didn't see it myself.
I'll give you the same warning I've given others heading to that part of France during the off-season: Check the websites of places you want to visit, because some of the smaller ones may not be open as many days a week as you would expect. You may encounter some 4-day weeks, or possibly even 3-day weeks.
I would use the train along the coast and just drive the middle and upper corniches (or rather, I wouldn't drive any of them, but I know you often drive in Europe). You'll probably save time with a car for any trip into the hills.
Try to find time for the relatively non-touristy Vence and Tourrettes-sur-Loup. I think TsL is very difficult if not impossible to manage without a car. In those two towns you'll be able to get photographs without other tourists in them, which I suspect will be impossible in most other Riviera towns, even in March.
If you have a particular interest in Picasso, you might like to fit in Vallauris, where he did much of his ceramic work. Vallauris is between Antibes and Cannes and doesn't get hordes of tourists. However, it's not a particularly striking town, architecturally. I also went to Biot, which is known for glass. There's a small glass museum there, and on the edge of town there's the Leger Museum, which I didn't have time for. Biot is atmospheric but won't make you forget Vence or St-Paul-de-Vence.
The tourist office on the plaza in front of the Nice Ville RR station was extremely helpful to me. It had brochures on some of the nearby peninsulas that I planned to walk around (Cap Ferrat, etc.). Don't pay too much attention to guide book references to the fancy houses along the coast. In my experience, they're nearly all behind high walls. Your best chance to see them is to look down on them from above.
There are weekly markets all over the place; I assume they operate in March. I never found an easy-to-use comprehensive list, so in the end I did a lot of Googling.
If you like gardens, I thought the best were the two in Menton and the one at the Villa Ephrussi de Rothschild. I didn't go to Monaco, though.
