I'm with Bandit_Driver 110%. But only if the plan is to eat breathe and sleep flying. Aaaand you find a CFII willing to instruct in a personal plane.
Going that route I figure it'll take about 20K in gas and instruction costs to 300 hours which would hopefully get you your CFI. If you negotiate a fresh annual on purchase and fly as much as you would at ATP you're only in it maintenance wise for oil changes and hopefully a couple grand in incidental maintenance. Worst case scenario, you're stuck with having to do an annual/IFR cert your self and figure 5K for a nightmare annual and you're in at 30K to CFI before purchase price adding in tie down fees and the odd quart of oil here and there. Find a decent IFR certified 172 for 39K and you're at bare minimum ATP prices. Figure you price the Skyhawk to sell ASAP when you're done and even then you'll get 25K back minimum.
Keep in mind my numbers are based in experience owning and flying 10 years ago, but there is a significant savings to be realized as older airplanes hold their value. I had a 1966 C-150F with dated gauges, one VOR, a flip flop COM mid time engine and about 7/10 interior and paint. I bought it in 2002 for 16K and that same airplane today would sell for the same price (if not more). Add 300 or so hours to the engine and you'll be looking at a loss on sale for 1 to 2 grand, but no more.
If you decide to go that route I'd ask around to find the best A&P (mechanic) in your area (that doesn't regularly work on the target aircraft). Go speak with them and ask them what to look for TBO, SMOH, TT, etc and get them to do a thorough pre buy inspection on any prospective aircraft. I'd even stipulate in any agreement that the buy comes with a fresh annual inspection, you'll pay for the inspection using your guy and the seller pays for parts and labor on any corrective actions. Having a second opinion will pay dividends down the road.
The last thing is, it'd be a training aircraft. You don't need a fancy panel mounted GPS, the paint doesn't need to be perfect and you don't need the kind of seats you'd see in a Mercedes. Don't let any of that sway you. Will it fit the training mission is number one, resale value 300 or whatever number of hours down the road is number two. Your ideal plane is a mechanically and avionically sound relic from the 70's or 80's with faded paint and an ADF.