I'm in almost the same position as you. I have the Lenovo P300 30AH004MUS. See my logo for description.
So here's how it breaks down. Lenovo uses proprietary power supplies. OEM for the 280W PSU is Lite-On, and OEM for the 450W model is FSP. Lite-On is not reviewed. They manufacture fully proprietary power supplies. The 450W is a custom FSP Aurum 92+. It has full JP capacitors (Nippon Chemi-Con), and the efficiency easily reaches the 80+ Platinum certification at ~92%, ±2%. The efficiency was one of the best I ever observed. However, at $114, there's 60mV ripple on the 5VSB and 12V rail, so it's a dealbreaker.
You can get a 24-pin to 14-pin Lenovo proprietary adapter, but that'll require modular PSUs. Otherwise, you would be stuck with a 24-pin PSU. And that won't work with the 14-pin adapter, making it redundant. So you'll need a fully modular PSU. There's no 500W PSUs on the market right now that'll allow you to apply the mod, so you should go get a 550W PSU.
As for the graphics card, it should sit in properly. The Lenovo M93 seems to be similar to an ATX build. The card should fit in without problems.
However, don't go for the top-of-the-line GTX 970 since that'll be a water-cooled card. You won't be able to fit that loop into the case, since there isn't a mounting place for that.
Here's what I would go with:
PCPartPicker part list /
Price breakdown by merchant
Video Card: Asus GeForce GTX 970 4GB STRIX Video Card ($318.89 @ OutletPC)
Power Supply: EVGA SuperNOVA G2 550W 80+ Gold Certified Fully-Modular ATX Power Supply ($59.99 @ Newegg)
Total: $378.88
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-02-02 02:28 EST-0500