Are you looking for kits that you intend to build and fly as mid power as a stepping stone to L1? Or are you looking for a rocket to use as your L1 cert rocket, but can also be flown on mid power before the cert flight? There's not a huge difference --- it's mostly a matter of the ideal motor class range. I usually think of rockets as having an ideal range of about 2 motor classes. Outside of that ideal range, you might be able to fly a smaller motor for a marginal flight, or a bigger motor for a riskier fast and high flight.
If you have not flown any MPR yet, then you could try any of the Estes PSII builder kits. Most of them have an ideal motor class range of F to G motors. You could build and fly these as MPR rockets and gain some experience with composite motors and MPR materials and building techniques. If you wanted to, you could even do your L1 cert flight using one of these rockets, but an H motor would mean a very fast and high flight.
I'm building a rocket right now that I think is going to be perfect for the G to H range of motors. It's an Aerotech G-Force, and the kit was originally designed to be flown on single-use G motors, so it's perfect for that. I left out the motor block and cooling mesh, so it will have room for H motors. And I think that the rocket is large enough that using H motors will not result in an unreasonably fast or high flight. Sometimes peopl fly them on I's. This might be a good transitional rocket for you if you mostly want to fly G motors during the time you are practicing for L1, but it's probably a bit big for F motors.
Moving up the scale, you can find rockets designed for L1 motors in the H-I range that could be flown marginally on G motors. I don't have as much experience with those rockets, so I don't have any specific kits to recommend. Until you have your cert, you would have to be careful in this range. If the rocket weighs over 1500 grams at launch, it's HPR and requires certification, even if flown on a G motor.
Maybe if you can narrow down the primary use of the rocket and the range of motors you will mostly fly, then people can give you more recommendations. Also what kind of flights do you prefer? Fast and high? Or low and slow?