DVB-T signals are OFDM (orthogonal frequency division multiplexing), which means that you take your symbols, put them in a vector, put that vector through a discrete fourier transform, and get the time-domain signal you want to transmit.
Now, this is fine and all, but OFDM hence has a high Peak-to-average power ratio. Which means that your amplifier needs to be quite linear across large dynamic range – otherwise, you'll get nonlinearities, which means intermodulation, which means (in OFDM, at least) inter-symbol interference, which means symbol errors, which means bit errors... For means of PAPR reduction in DVB signals, this paper might be of interest. The takeaway from that paper is that in an easy 1% of the symbols (i.e. in 1% of your data carrying units), there is a PAPR > 12.
I'd say that infers a need for a spurious-free dynamic range of at least 20dB for your amplifier. Note that an amplifier rated for an output power $x$ will not be linear close to that power – take away around 9 dB, and you'd typically end up close to the upper edge of your linear range. That means that if you need 5W of power in the linear range, your amplifier would typically be rated $8\cdot 5\,\text W= 40\,\text W$. Note that this is really just a rule of thumb. How conservatively sellers spec their maximum output power or how much the amplifier is designed for maximum single-carrier or broadband transmission, or power efficiency, will play a big role. There's no way around getting some curve or numbers from which you can see how linear an amplifier is, over what range.
Now, the HackRF, being very universal, is not a spectacularly "clean" device, which means that your output signal might (will!) contain out-of-band emissions. You should probably counter that with an analog filter, which doesn't have to be overly sharp – just make sure no multiples of your center frequency and no dividers of it make it into the output.
Look for impedance matching. I'm not quite sure, but from the top of my head, the HackRF has a nominal output impedance of 50 Ohm. It's probably slightly different for different frequencies, but you'll not lose much with that.
Make sure the amplifier is not leaking back too much energy into it's input (input/output isolation). Rule of thumb: Max output power of HackRF - 3dB should be the maximum amount you leak back. Make sure not to start the HackRF in receive mode if the energy leaked into the connector would kill the sensitive input electronics.