Since you are "only"
looking to get around 500yds, a bone stock Savage Striker 22-250 would do that without a problem...and save you a bunch of money. I think Ernie made the 500yd club with a XP-100 in .221FB. I've made several 400yds and close to 400yds one shot kills with my Striker 22-250 and 50gr V-Max.
With building on the XP that you have, you will be looking at getting the bolt face opened up to accept the larger case head. That will also involve the addition of a new extractor (Sako style is recommended). To help, I pasted a list of prices of work that I recently had done to an XP that I had built into a 7mm-270WSM...most of the items are things that you would need to have done to you gun.
Lilja 1-9 stainless barrel...................................$295
>
> Receiver accurizing............................................$175
>
> Fit bolt with Sako extractor................................$120
>
> Adjust bolt stop(if needed)...............................$25
>
> Machine Holland Comp recoil lug..................$95
>
> Fit and chamber barrel......................................$175
>
> Matte finish barrel...............................................$30
>
> Holland QD or DE muzzle brake.................................$150 installed
>
> Inletting and bedding.........................................$85
>
> Trigger
> tuning......................................................$40
Other than the brake and matte finish, most of these items are going to be costs incurred for the build. You will also need to get a stock. The Zytel will not work with a heavy barrel. That will run you in the neighborhood of $335 (shipped) from HS Precision. I got one on the web used but still paid $175 and it took months to find. Ernie has switched to using McMillan stock but I am not sure what thye cost...but I can not imagine they are going to be any cheaper. So between a stock and a barrel you will easily wack MOST of you budget.
You will also have to consider optics unless you have something already. For PD shooting I would go with a rifle scope. I don't do any of that kind of shooting so I only use the Burris 3-12x32 w/ either target knobs or the BP reticle.
Mounts and rings...as much as I wanted to spend the money on a steel Farrel base, I only had funds for a Weaver when it came time to plunk down the cash. So far it is holding up fine on the WSM XP-100. My other custom XP's (.222 Rem and .222 Rem Mag Imp) also have Weaver and are just fine. For rings, I use the Burris Signature w/ inserts ("0" in front and .020" in the back) to gain enough elevation to allow me to dial in for ranges to 500yds...and hopefully beyond when I get a better rangefinder and longer place to shoot.
Bolt release...I only have one gun with a release but I do need one on the WSM XP. Not sure of the name but the one on my .222 Rem Mag Imp is sold through Sinclair's.