Redhat makes an excellent point Pieritz. If your Spitfire AM output is rated as 100mW PEP and the linear has a 13-14 dB gain figure, the exciter will drive it to about 2 watts PEP output. If the exciter is rated as 100 mW carrier output, its PEP will be 400 mW, if fully modulated. This will drive a linear with a 13-14 dB gain figure to about 8-10 watts PEP. The overall point is to ensure that your linear amp will safely develop the power that its being driven to. If not, POP goes the MOSFETs. Of course, even before that point is reached, you will be scraping off more and more of your modulation envelope, the more you overdrive the linear.
Also what Redhat said about producing heat and power wastage is absolutely true. You will have to decide whether a linear is really the way you want to go. Although I have dedicated rigs for 43M and 48M, I use linears a lot to get a pirate signal up on the higher bands. I find it easier to build and tweak many band-specific exciters and low pass filters (and dipoles), and bring the power level up with a broadband linear. I can easily run a couple hours from a 90aH Marine Battery. It works for me. Only you can know if a linear is the right way for you to go.