Newby choosing a bike

I know this is a common question, so here are some specifics. I live in Lake Tahoe and ride a mountain bike. A long time ago I was an Ironman triathlete. I am 46, in decent shape and hope to get in very good shape again. Cyclocross just caught my eye and I think it would be a lot of fun. I will ride mostly for fitness, longer rides on fire roads, some single track, etc. I'd love to do some racing for fun, camaraderie, and to push myself. I have ridden high enough quality gear on the road that I am nervous about buying a bike with say, 105 components. I also don't want something very heavy. I don't know if these concerns are well founded. I want  to ride a bike that i'll be psyched to get on. I would like to stay closer to the $1,000 range, and will be fine buying used. None of my local bike shops have cyclocross bikes, nor do they know much about them. Can you share your input, please, relative to what I may look for and be happy with, realistically? I would also like to be able to change to road wheels and do some road rides when it is safe to do so around here.
A quick note: I am 5'10", 170 lbs and ride a 56cm road bike. Should I be looking at a 54cm in CX?

Thanks for any direction!

Jon

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    Kris Fleming

    105 is MORE than serviceable as a groupset.  Having any group set-up and tuned properly is what is paramount in all cases.

    As for fit, do you know the rest of the geometry on your "56cm road frame"?  That is important.  If you know the Effective top tube length (ETT) and seat tube angle (STA) on your road bike, you can figure out reach.  I'd be looking to have a very similar reach between my road bike and CX bike.  Maybe 1cm shorter on the CX, but that can be accomplished in most cases with a slightly shorter stem if needed.  The issue is, CX bikes usually have a slightly more relaxed geometry (at least in the sizes I typically ride), so being able to do the calculation, to compare apples to apples is very helpful.  Basically 1 degree difference in STA effects your reach in relation to ETT by 1 cm.  So a bike with a 52cm ETT and 75 degree STA will have nearly the same reach as a bike with a 53cm ETT and 74 degree STA, etc.  I'd probably start with frames that are named 56cm and compare the geo from there.  

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      Jon Paul

      Kris and Kenneth,

      Your input was helpful. I ended up getting a 2015 Giant TCX Slr 2 in the Medium size. I found it used on Craigslist from someone that works in a shop for a good price, and only an hour from home. I may need to play with the stem length a little, but I think the fit is good. I did an hour ride on single track and fire road with lots of mud, wet sand, rocks, roots and step hills. One of the most fun rides I've done. I was impressed with the cross bikes ability to handle the terrain and slick conditions. I'm excited to try some events, but for now I'll be trying to get in shape in between snow storms.