Kevin -
The first thing is you have to decide what you want at the end of it. To me 15K is a lot of money and several of my CBC friends have got rid of CNC machinee centres for a lot less than that. I don't have the space, unfortunately.What are you seeking to do with the machine. You have talked about the firearms work etc, but that doesn't seem all that much (to me), unless you are a registered dealer.
A CNC machine has two advantages. One is speed - but in actual fact if you add all the time it takes to program the thing, for a one off you could be better doing it manually. Having said that, I prefer to do the program in the comfort of my "office" (front room) and then transfer it to my workshop computer to run it, rather than be bunched up over the lathe all the time. The second is repeatability. If you are machining a number of the same parts, then CNC is the way to go. I think there may be a third - in that programing is easier to me that machining. I can program the machine, test the program, and then be confident the machine will cut it. Doing it manually, I could make a mess and spoil it.
My lathe is much faster now than it used to be, particularly on the rapid non-cutting moves (and it doesn't waste time thinking about it)
Cutting is much the same, you have to use the same cutting speeds anyway. To achieve this, I have not altered my balls screws, or anything, I have just installed stepper motors. Mine could be wired three different ways - see an earlier post - now thety are wired in parrallel, with the Geckos, they fly.
If your lathe is in reasonable condition then a regrind might be all that it needs. How is your backlash. Mine is a lot, but Mach 3 copes very admirably with it. Cutting ball shapes you cannot see the join.
If it were me, I would have the machine reground, and delivered back. Quite frankly, if you are a reasonable engineer, you can do the rest yourself.
If you are in UK - (you talk in £'s) - then purchase a motor from Arc Euro Trade. They sell a 650Ncm motor for £50. This is 3 times the power I use (my lathe is a large Warco combined lathe/mill - and I use the 220Ncm). They should drive anything and your machine looks like my friends Harrisson (of a similar size anyway). I would buy one motor for the time being, and see it's capabilities. Unfortunately I do not know of any simple way of calculating what power you will need, nobody on this forum has come up with any ideas.
To power this motor you need a stepper driver. Buy single units - do not be tempted to buy a three in one - if one breaks, it's easier to fix. If I were you I would buy Gecko drives - American - very good and the standard Gecko has the voltage and current capabilities to drive the 650 motor. All you have to do is fit them to your lathe. My photograph on the earlier post shows the one fitted to the lead screw. Yes, I have used a toothed belt drive, these are cheap, they transmit the power without slipping, and the range of sizes is enormous. The advantage is - if you fit your motor - say with a 1 to 1 drive - and you find you need a bit more power - then fit different cogs and you can scale up your power ratio (for a loss of speed I admit) - but you will be able to find the ratio you are comfortable with that gives you enough power - with a speed you can live with. Merely remove your handwheel, fit a toothed cog, mount the motor in a suitable place and then buy the correct sized belt. My belts are 9mm wide, 5mm pitch. On your machine I would use the next size up. These should be available from any bearing supplier - and haggle the price - they give up to 90% discount - I kid you not. I WILL fit covers on them sooner or later (probably later knowing me).
I don't have a problem with speed on a lathe. These lads that have aircraft carrier flight decks for their router tables like the thing to travel at the speed of light. I got mine up to 40 inches per minute ( with a 3 to 1 reduction) so with less than 20 ins travel, I could be up and down in no time. I have cut the speed to 20 ins so that I trade accuracy (less chance of missed steps) for speed.
If your leadscrew is in reasonable condition, then don't change it, unless you really want to - but I would try with it at first and see the results. As you gain confidence, you will decide how far you want to go. For the sake of simplicity I would use the same motor on the cross-slide - which is why I said only buy one to start with. A 650 motor should certainly cope with the cross-slide - and you can probably use a higher gear if you wanted, although again - speed isn't everything. If the motor is too small for your leadscrew, use it on your cross-slide. If it's OK then but another one.
For £500 you should be able to buy 2 motors, Gecko drives, gearing and belts to get started. The only other thing you will need is a power unit. For this, you have little option but to buy a professional power unit. The motors are rated at 5 volts - steppers are "over driven" so a 30 volt power unit able to give about 20 amps should do the job. This may be expensive, but we are still well short of the 15K
Don't get the idea that these "machine shops" can do any better at retrofitting a lathe than you can. All they do is cover it up so you can't see what they have done - and charge you a hell of a price for it. You can buy ALL the parts for £1000. Yes you could spend a lot more I am sure, and other people will probably advise you to buy something different. Certainly for £2000 you can get the best - certainly to do a two axis lathe - so where does the other 28K come in.
Have a try - very little will be wasted and you will gain a lot of knowledge, even if, in the end, someone does most of the work for you.