First take heed of Audioguru's comments above.
Making a TV transmitter amplifier is not as simple as choosing a few transistors. The device you mention is nowhere near adequate to amplify an analogue TV signal to 5W without significant distortion products being generated. You might get 0.5W perhaps 1W if well designed, and it won't work at UHF.
If by a standard rf modulator you mean something that is plugged into a TV set than that will not give you a signal that will be acceptable to amplify to 5W an put into an antenna. If it is a professional modulator with VSB filtering and up-conversion then the design of the power amplifier that follows it is not going to be trivial even at 5W O/P if the signal quality is to be maintained.
I would not want to discourage anyone from learning how to do it.
Look up linear power amplifier design, there are plenty of application notes for TV power amplifiers, NXP would be a good place to start. From there you should be able to find out about input, output and inter-stage impedance matching.
Then look at power distribution and bias supply design, this will be important to maintain signal quality.
Another important topic is filter design, you will need a good low-pass filter on the output of the amplifier.
Before you build it learn about EMI (electromagnetic interference) screening and filtering or you will build yourself a power oscillator.
Lastly, take heed of Audioguru's comments above.
>> i've heard that class C rf amp cannot be used for TV transmitting purposes,
They can, I've done it, it is very difficult; you need all the above skills and more to make it work.