Frame differences between 700c and 29er?
Since both the bikes you link to are mountain bikes, you seem to be primarily concerned with wheel size or frame size. 

But the two wheel sizes are the same = 700c is, more or less, a rim size, while 29" is the outer diameter of the tyre (except that obviously you can get different width "29er" tyres and that affects outside diameter). That confusion is why tyres are now actually sold by ISO rim diameter - specifically the bead seat diameter. If you get that right the tyre will go onto the rim (it may not fit into your frame, or it may be far too fat for the narrow rim, but it will go on).

Frame size matters when you convert one of those to ride fast on sealed roads, you're going to need to do more than just switch the tyres. Most obviously, the 29er is generally lower than the cross frame so when you put the seat and handlebars up to get a better position for fast riding you may find that the 29er can't do that with the stem and seat post that come with it. So you'll probably have to buy a rising stem and possibly a head tube extender, and (less likely) a longer seatpost.

The gearing is also different, 48-11 top gear on the cross bike and 42-11 on the 29er. For both bikes you're probably going to want higher gears, but for the 29er the jump is going to be significant meaning you might need to change the front derailleur as well. I suggest allowing for a 50T or 53T large chainring, which on the cross bike is a small change and may not be worth while. It depends on how fast you ride and pedal.

Both frames are aluminium, just like most mid-range road bikes. I don't think that matters for what you're doing.

On that site the touring bikes (TREKKINGOWE?) would probably be an easier conversion to what you want, but I suspect you might prefer a cyclo-cross bike (drop bars, disk brakes, slightly heavier/stronger than a road bike). Cyclocross bikes are or were made in Poland, but the website for 'Scattante' is gone. Here's a German manufacturer instead :) They won't carry as much weight as a touring bike, but as a result they're lighter. And they're usually designed for more speed.