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Can you live with loss and do without isolation if so, the simplest solution is three resistors either in T or Star formation.
If you don't need to go too low in frequency, > 1MHz or so, a reactive combiner will do the trick. Lower loss but gives isolation. The ammount depends on the match of the load
By using a high frequency opamp you could even use a standard 'video' type mixer circuit. You will need to check the stability of the opamp at low gain though. A 400MHz opamp should work OK.
For topology and design equations see this site, it's the first one I found with diagrams and equations. It also ives a number of different topologies that may be useful to you
I would use 3 + 3 turns on the output transformer for a lower frequency response.
For the core any of the cores on page 45 of the fairite catalogue will do 302 or 702 size, either type 43 or 73 material will do.
For the frequency range you are using the capacitor is not really needed, a 4.7pF will improve the input match but at 70MHz you are hardly going to notice. https://www.fair-rite.com/
If you don't want to make a splitter you can get them quite cheaply from Minicircuits
As for an opamp try Maxim MAX4108. Check that the slew rate is going to be adequate for your output signal.
If you need and RF amplifier again Minicircuits do a number of wideaband gainblocks quite cheaply. They are simple to use, but do take more current than a discrete tranisitor amplifier.
Assuming you are only operating at low power, a few tens of milliwatts then a small core will be fine.
I usually use the circuit in figure 5. You can use the turns given, it will work, but in your frequency range I would use 3 + 3 turns instead of 2 + 2. for the split.
Ignore the capacitor you will not see any improvement by using it.
For core type and number of turns see post above.
Fairite 2873002302 or 2843002302 will work.
For wire size so long as it will fit the core you will be OK, somethiing aorund 28 AWG or SWG will work fine. It is not that critical.
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