The kits are tempting. Just remember you need more than the MOSFETs, heatsinks, relays. For example, you are going to need to buy or build a lowpass filter board capable of covering each desired operating band. You might want metering. That is another circuit. You probably want some type of protection, say for severe SWR mismatch. That is yet another circuit. Toss in a power supply, too.
I like the idea of building. It teaches electronics skills. Though if perhaps more interested in operating, compare what you are going to spend building, not mention the potential hassle, against buying. Check QRZ and similar for used deals. A common example is an used working AL-811(/H) with good tubes, and especially if swapped to 572B tubes, is a popular affordable HF amp for a few hundred watts. Drive it reasonably, not for absolute PEP output, and it probably will last for years of service.