it also depends on what your goals are, and what you plan to do in the hobby. (And how long you want to spend in it!)
You will soon be buying these parts.. depends on when "soon" is..
Buy a 38mm case now & the RAS system soon after. That covers a fair amount of L1 motors. (and it'll soon pay for itself, rather than continually buying DMS motors) Eventually you will buy another [longer] 38mm case. This new / 2nd case can be used for your L2, in a bird that has a 38mm hole. (Why buy a 54mm bird now?)
Your next rocket may come with a 54mm hole. Now get the adapter. You can then use a variety of higher end 38mm reloads with the 2 cases (and likely 3 cases now!)
Now you can buy rockets with either a 38mm or 54mm hole. Soon you will buy a 54mm case.. And so on. This is what I did, over about a 3 or 4 year period.. Buy a piece of hardware each year. Budget to take advantage of the yearly sales (Wildman Black Friday for instance - if budgeted right, you can have a year's worth of HPR motors for 20-40% less than buying them individually)
If you fly L1 regularly, you will get into RMS motors. You will also have the budget for $100 a day for motors. Miss one week-end, and buy hardware instead. L1 motors are typically 29mm & 38mm. Once you get your L2, you will soon fly 54mm motors.. L2 rockets can be flown on 38mm & 54mm motors..
It's not a race.. Remember, that guy down the street with the sweet '68 Chevelle started with a basic junker, and has slowly added to it over the years.. (One / 2 years was just the paint!!)
Watch the yard sale forum. Cases come up all the time (but go fast!) Also, take part in the annual TRF Secret Santa.. I received the whole 38mm CTI case set one year, and some 29 & 38mm cases other years..