For the DSP purpose, a Helix DSP would be the one with the best tuning software accessible to you. However alternative in the same budget range vary from Alpine h701 and Mosconi 4to6. I would vouch for Helix DSP any day. Great quality and superb tuning software.
As far as the amplifier is concerned, in the class D category I would second @bilalicious. The PDX F6 and the PDX V9 are the two best class D amplifiers I have heard to date. Every intricate detail in the music can be heard with amazing power and smallest footprint. I would rate is the king of class D give you find a piece in good nick (brand new locally are too exp). V9 would power your entire setup. However F6 looks more feasible for you midbass and subwoofer duty.
As far as subwoofer is concerned, the best budget or mid-tier subwoofer I have experience and owned to date is Infinity Kappa Perfect 12VQ. Nothing comes close to it given you can find it locally at a decent price. However if HX300 is in easy reach of your budget then go for it.
All the best for the install sir 
