The first step in choosing a load center for your property is to perform a load calculation on your electrical system. The average handyman will not know how to perform this load calculation. You could purchase a National Electrical Code handbook and do it yourself. If you have no experience in calculations for a system this size, I recommend you hire a professional electrician to perform this vital step. They should be able to perform this should only take two to three hours. Be forewarned, this step is a necessity to cover insurance and code requirements.
Next, I would have you check the size of your feeder wire. This is usually the aluminum wire that travels from your meter base to your current box. The most common way to figure this out is to remove the cover to the existing load center. Typically, a home will have a 150 amp panel, a 200 amp panel, or a combination of 2 or 3 of these panels depending on the size of your electrical service. A 150 amp panel requires a four wire cable consisting of three (2/0) wires and one (1/0) wire. These would be aluminum conductors. The 200 amp panel would require a four wire cable consisting of three (4/0) wires and one (2/0) wire. The existing wire should have a label somewhere on the cable.
Now you need to count the number of circuits that you want to power with your new load center. Sometimes, you will be able to acquire panels with approximately 32 slots for your circuit breakers. Usually I would suggest a 42 slot load center. This will allow for any additions that you may want to include in your residential electrical remodel. Be sure that your master electrician includes any planned circuits in his load calculation. Failure to include the added circuits could possibly cause a circuit overload. Circuit overloads can be a fire hazard. Ultimately this miscalculation will cause overheating in the electrical panel.
Generally, I never buy anything without considering the quality of the product that I intend to purchase. Most of the load centers, that are for sale currently, are equal. You may be able to find value packs. This purchase includes some random circuit breakers with your load center. Also the price of the remaining needed breakers can be a determining factor. The width and length may weigh on your decision. Most of these load centers are 14 1/2 inches wide. This enables the installer to screw the load center directly to the interior walls. You should not have to modify the supporting lumber. I installed over 100 of these in the past few years. I only had to modify the structural lumber once. This installation was in an apartment complex.
Look for more directional items on electrical installations if you are unsure about moving forward. Do not forget to acquire your load calculation. When removing the existing electric panel, do not contact the live busses that are existing there.