I'm only familiar with the Bachmann Spectrum 2-8-0, which is an excellent-running locomotive.

A locomotive with wheels made to NMRA RP25 recommendations should operate without problems on Code 70 and Code 55 rail.  If handlaying, you need to test a section to be sure the spikes you are using are not so big that they contact the wheels.

My experience with Bachmann DCC decoders is that they are functional but that aftermarket decoders will usually offer more precise control and some will have more functions available.

My advice: buy a locomotive, get some track down, and run them for a while before adding other locomotives into the mix.

All of the engines you mentioned are pretty good.  The worst of the bunch is the 0-6-0 - while decent and a lot better then the older versions, it is not Spectrum quality.  I would also suggest you look at the Spectrum 4-4-0 and 4-6-0, as I have seen both going on ebay for $70-$100 and I find them to be of excellent quality.

If you want the put the engines on the track and have them run, the DCC engines are fine.  If you want better quality decoders, be prepared to do a little more work.  While there are drop-in decoders for these engines, Bachmann models are notorious for being wired improperly (for example, I had one with the orange & gray motor wired backwards), so many people completely gut all wiring and electronics and start from scratch.  This is especially common if you want to install sound, as the small tenders on these small engines do not have a lot of room, so the room taken up by extra circuit boards can be an issue.

As Joe said, code 70 should not be a problem.  Code 55 might, depending on the spikes you use. A word of caution - if you get heavily into steam models, you will likely start looking at other older models before long.  Many of those (such as the early IHC, AMH, Rivarossi, Bachman, Mantua, etc.) have flanges much deeper than RP25, and they will definitely not run on code 70, let alone code 55.  I have an older IHC pacific which does not even like my code 83 in a few places.  Unless you are prepared to turn the flanges down, or you know positively that you will never run any of those engines, I would suggest considering code 83, or at least code 70.

Right then. Code 70: what rail spikes should I use? I see in the Walthers catalog that ME has different sizes.

I'm Buying only new models equiped with DCC. This way I hope to stay clear from the older models.

Ebay is not an option because most private sellers do not ship outside the USA and if they do the import tax is killing any bargain. 

Use ME small spikes.  The micro are really tiny, and usually only useful for finescale or N scale applications.  You should have no problems with Code 55 spiked with ME small spikes if you run RP25 wheels.