I'm very new to the world of bicycles, so apologies in advance. I have a second hand road bike built (by a professional) with spare parts of different components. The rear derailleur is Shimano Acera, the front derailleur is Shimano Tiagra. I have Shimano Tourney/TY SL-TX50 Thumb Shifter Plus - Silver, 7 Speed (or something like that). It also has caliper brakes of unknown brand/model. I am looking into slowly upgrading the components and was looking into upgrading the brakes and I am keen into changing the shifter and brakes for brake lever/shifters where it is all together. However, I am not sure which brakes or which brake lever/shifter I should choose to work on my bike. Should I change either the back derailleur (or front one) first or can I first change the caliper brakes for better ones as well as changing the brake level/shifters and at a later point change either the front or back derailleur? Could anyone suggest anything?