As others have said try them with the demo versions of Pianoteq - it's a personal choice.
First things first : you can add instrument packs any time after you buy Pianoteq so you're not locked in to just one set or another. There's no real financial penalty for this except for the number of instrument packs you get with the different versions of Pianoteq. All of the versions produce the same sounds from inbuilt presets. However Standard comes with three instrument packs and Pro with four. Standard and Pro support microphones as well and have better customization options, so I generally suggest Standard is the best value if you're a serious buyer and can make the initial investment. Pro, IMO, really does require "pro" skills in the form of sound production and engineering to exploit or specialist goals.
The free instruments are very nice and have been updated since their initial release. I would not recommend someone serious about historic instruments relying exclusively on them. I'd suggest one modern grand is a minimum choice to make - I personally prefer the Steinway B or Steingraeber over the others but this is a very personal choice. In particular transcriptions of some much written for grand piano sound better on a modern instrument than on an old pianoforte and pieces written for pianoforte (e.g. some pianoforte transcriptions of BWV 1004) sound great on pianoforte and just wrong on a modern grand. I think at least one modern grand is necessary for modern styles.
Note the Klemsegg 2 pack which I have in this regard. It has a Bechstein 1899 (modeled from one owned by Gilbert O'Sullivan of that means anything to you) and is an "almost" modern grand piano sound. It also contains the Frenzel - a personal favorite of mine which has a kind of old-meets-new sound that's very nice to my ear. It has two "more traditional" pianoforte instruments which are really very nice especially when played with the appropriate music. Getting that pack is one I could personally recommend and certainly check them in the demos. Four excellent instruments for the price of one !
The harpsicord in the new Karsten pack was an immediately gorgeous sound. I'm no harpsichord expert, but it's lovely to my ear and yet another instrument "on my list" of possible future purchases. The Weimes in the same pack sounds quite interesting too. I appear t be the only person who finds the square Steinway uninteresting, but it's all personal choices. :-)
The free Erard and Pleyel are similar to the Bechstein 1899 (perhaps slightly less polished sounds ?) in that they're "almost modern" to present day years. If you're used to hearing a modern Steinway B they sound old, if you're comparing them to a Pleyel 1835 they sound modern. I would personally not classify them as modern, but in the same period as the Bechstein 1899. I'd suggest not relying on them as a modern piano sound for that reason.
I'm not qualified to really comment on the free harpsichords in any technical sense. They sound good to me is about all I'd say. If in doubt I'd suggest starting with them and considering other packs for later purchase. The Hans Ruckers pack is beyond my knowledge to comment on - I can't say what I've heard makes me personally rush to buy it, but then I don't really have an interest in harpsichords and would not really use it myself.
So I'd recommend at least one good modern grand (if you need a modern grand at all ?) and Klemsegg 1 or 2 or Karsten if your interests go in that directions. The free instruments are a great bonus but if you don't need more than one modern grand then pick an older instrument pack or packs. Don't think of the free Erard and Pleyel as modern grands - they're "almost modern grands" IMO (but good in their own rights).
StephenG