My 2 cents worth:

Our family fleet includes an '87 245, a '90 240, a '91 245, a '98 S70, and daughter's boyfriend had a V70.

The '98 S70 has been a great car. Wonderful seats. Has required a little more service in its first 125k miles than any of the bricks. Power steering has always been stiff and now leaks. ABS control module failed last year. Interior plastic is even more fragile than the plastic in a brick. All minor problems.

The V70 was a beautiful car but suffered from the (apparently common) problem with sucking air though a defective O-ring in the oil pickup. Sounds like an easy repair but it isn't. We were just about to tackle it when he rear-ended a Mercedes and totaled the V. Thankfully, no injuries. If you are going to ram two cars together, hitting a full-sized Mercedes with a V70 is a good choice.

Your '91 brick with only 115k is still a great car. If you mean that it is diluting the oil with gasoline, try unplugging the fifth fuel injector. It is not really necessary anyway unless you live in Canada. You are sure that the fuel pressure diaphragm is not leaking?

I used to say that I would get rid of my '91 when it hit 250k miles. Now I am there, and I am thinking 350k miles.

Have you also posted this question at the V70-XC70 forums? We 240 folks know our 240s, but how many of us also have a V70? You'll get a broader range of contact people who are familiar with the V/XC70 cars at that models forums.

Frankly, I seem to remember hearing that there's a few problems with that car over a range of years, possibly including '98.
One is with an electronic module that controls the throttle -- in this case, it's a "drive-by-wire" control of the throttle. If you have a mechanical linkage to the throttle body, then your car predates the problem; if you don't have a mechanical linkage, then it's possibly subject to this defect.

A second depends on whether your prospective V70 is AWD. If it's FWD, then again no problem; it's the AWD cars that have a problem with the bevel gear failing. If it's AWD, realize that you cannot use a doughnut spare tire for more than just a few miles (seriously!), and you must rotate tires often; forgetting either 'cooks' the bevel gear!

But for expert (or at least experienced) folks critiquing the V70, be sure to check their forum.

Good luck.

Fix the 240. With that few miles, htere's something trivial wrong with it.
V70's are great, I have a '04 V70 2.5T that I drive 7k miles a year. I drive my '91 245 30k mi /yr and it runs just as well as the V70.
Get the V70, but keep the 240 around and figure what's wrong with it.
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'89 245 sportwagon, destroyed by hit & run driver, RIP. '04 V70 2.5 T Sportwagon, 12k mi and '91 245 5-speed, 209k mi, replaced the '89

For what it is worth, I would go with the V70. I have worked on both 240s and 850s/V70s, and the 850s/V70s are much easier (at least for me). The one draw back (at least from my vantage point) is that the modern Volvos are front wheel drive, while the 240s are rear wheel drive.

Too obvious to suggest fuel pressure regulator failure of temp sensor failure, but I have not followed your thread. The S70 is a great car, but the brick is the safest, easiest, greatest car ever. I'd chose to have both. Hey, I do have both.

For sure it is less diy friendly than your 91 240. You'll be more dependent on a professional technician. Here on RWD we fix gasified oil problems daily.
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-K (hope springs eternal)