Assuming that you are the only Employee at your Home office, This is what I recommend.
First thing is BACKUPS Most Small Business's Forget about Backups and assume that a PC will run forever and not have any problems.
I suggest an External USB drive They are Cheap and reliable. For a couple extra bucks from the basic ones you can get one that does 1 touch backups (push of a button.)
Next Keep your files backed up, Either use NTBACKUP (if you have XP Home you have to Install it from the CD) SYNCBACK or even Brief Case. My Main Point is there is more than 1 Place that your Data Resides. (At the End of the day) Even if you have to POP a DVD-RW and copy files to it once and a while.
DATA is what your Company Runs on....
If you have more than 1 Person Say 4 you may want to get Small Business Server 2003 (I have information on my site or you can search Microsoft for more information on this product)
As far as a PC, I always Suggest to my Clients to Get Middle or Just above Middle of PC's (Meaning Don't buy the latest and greatest because you pay extra for that and buy a little more than you think you need.)
If you are worried about the system going out of date in 8 months.. then you shouldn't buy anything. (This is just a fact of life and it doesn't matter what you buy it's going to happen, but putting it off won't make things better, Buy something you are happy with Now and you will think you will be happy in the next year.)
As far as XP Home and XP Pro, I'd Recommend XP Pro, If your business booms in the next year you will probably upgrade to something like SBS2003 There are other reasons why XP Pro is better, (Able to Encrypt files/folders Remote Desktop Access etc.)
As Far as System Setup...
I'm assuming that you have A website and you have DSL and or Cable Internet, and people can easily get information about your company that way?
Recommend at least a 2.8Ghz (You may Not need it now, but for the prices they are the best dollar value)
Dell usually has some on sale
Please Get at Least a 19 Inch Monitor.. Why?
Less Eye Stress. and in your business the less stress you have the better. Besides 19 Inch monitors are relatively cheap nowadays.. If Desktop Space is Premium and you MUST have a LCD display go a head and buy it, but please get a least a 19inch...
Keyboard: Doesn't matter what ever you like, Ergo, Etc. (I don't recommend Cordless unless you must have it. because if your batteries die and you don't have any, you can't work.) Mouse Same Corded, but doesnt matter if it's USB or PS2 Type Get OPTICAL (no Ball Mice) and wheel (Scroll)
If Possible get a Dual Layer DVD Drive in it.
(It may be cheaper to buy one and put it in after you get the PC than buying one from DELL or HP)
Dual Layer DVD blanks are Expensive now but they will come down. (A Dual layer holds about 8.5GB and a Single Layer about 4.7 GB of data)
Since you are construction you may also want to consider a Laptop. (and WiFi at Home for your HiSpeed internet.)
Laptops today are not the same Slow things they were before, for under 2k you can get a powerful laptop that can do everything most desktops can.
They even have ones that are water resistant/dust resistant etc.
In My job (IT Consulting) a laptop is a must)
I have some Clients who have a business like you that use a laptop to give estimates to clients and do a print out right from their truck. (they don't have to go back to their office)
I hope this helps
Specializing in Small Business Server 2003
Portland/Beaverton OR USA