Here is the order of things I would change staring with those that give most bang for the buck.
1. Use a yeast starter to ensure you pitch an adequate amount of healthy yeast. Only requires a flask, some DME, and a day or two planning ahead
2. Get a fridge with an external thermostat, or some other way to control your fermentation temperature so you can ferment at a constant temperature appropriate for the style you are brewing. Will require fridge and external temp control, but causes a bifg improvement in quality of beer
3. Move to a full boil. Will require a big kettle and wort chiller
4. All Grain. Requires Mash tun (or to minimize the amount of equipment to buy try brew in a bag) Also requires capability to do full boil