Bristlecone Pine (pinus aristata) nursery stock

I purchased a nice bristlecone pine that I think has great long term potential for bonsai. It has a nice thick, curvy trunk. Unfortunately the strength of the tree is mostly at the top. There are already nice back budding along the trunk but again the top is where the strongest growth is. I'm not sure what the first step I should take. Repot into better soil and let it just grow on its own for a season? Leave it in nursery pot and focus on balanceing the trees strength?

Have you read the picture posting tutorial? 

https://ibonsaiclub.forumotion.com/t243-tutorial-on-posting-pics

I am assuming you want to use the lower branches in the tree's design... You are right to be concerned about your trees top vigor. Ryan Neil told our group that the first thing he does with a pine, in any season, is balance the vigor all around the tree. This means pulling or cutting needles, in your case, at the top to "match" (I think) the growth of the lower branches with less foliage. Do not pull or cut any needles on the low branches with minimum foliage. I think you can safely cut the new candles at the top as well. I am not a pine expert, but this is what I would do if I owned your pine. Do not repot it now. I'm sure others will offer similar advice. Hope this helps.

Having not worked with bristlecone, my grain of salty advice would be similar to Todd's except that I would recommend rubbing off buds as they form in the fall and winter on the upper portions of the tree in unwanted areas.  I've found this to be an under utilized technique for balancing growth with a minimum of disruption of growth generally for pines.

Thanks to you both. I removed the candles on top of tree today. I will continue to pluck new growth at top.

As these are very limber trees you can wire and change its shape pretty drastically including a good part of the trunk I believe. You might consider a literati, windswept or cascade style. Also before making a major life choice for the tree check where your major surface roots are and how symmetrically or not these are to decide a final angle in the pot to bring the most to the surface for nebari. Wiring will need to be left on for quite awhile as the branches are so limber and don't set up as with hardwoods like beeches or maples. Best of fortune with it.

Many trees are prone to excessive top and branch end growth. The goal should be to balance this growth. This means inhibiting growth when it is strongest (top and branch ends) and strengthening it where it is weakest (interior and bottom). This is done by removing the strongest candles, needle plucking to balance foliage everywhere, and fertilize the heck out of it while it is still in the developmental stages.