How long should my Win7 laptop last?

I agree with Strollin :)
An external enclosure is a great option for a laptop
Difficult to get programs installed on it but it great for personal files and accessing them on it,

Samsung 850 evo's are good
I've in the past also used Crucial ssd's as well they are very reasonably priced
With a 500gb ssd from what you said yes you could do without the external hdd for a while if 300gb is what you do use.

I have I believe 4 Crucial ssd's I got MX100 series but the BX series is also very good and a tad cheaper than Samsung ones.

We can help you swap the os from the hdd to a new ssd later

I have used ssds from Kingston, Crucial, Intel and Samsung but like the Samsung's the best. I don't think you can go terribly wrong with any of the brands. In the early days of ssds, some of the controllers used by certain brands weren't as reliable as others but I haven't heard of those kinds of issues for several years now.

I'd have to win the lottery to be able to buy a Intel ssd :)
Outrageously priced.

No more so then a Samsung 850 Pro :)

In fact this Samsung Pro is downright cheap, in my opinion.

SSD 850 PRO 2.5� SATA III MZ-7KE256BW | Memory & Storage

What Intel are we looking at ? Same size roughly, in the 730 series ?

Intel 730 Series SSDSC2BP240G4R5 2.5" 240GB SATA 6Gb/s MLC Internal Solid State Drive (SSD) - Newegg.com

The Samsung beats the Intel with size and price :)

And the Samsung 850 EVO 250 GB ???

Just look for yourself, can`t beat it

SSD 850 EVO 2.5" SATA III 250GB | Samsung Solid State Drives

Good to here I can't remember which one now
But as far as ssd's go might as well get a 500gb they are dropping in price nicely and it gives you a lot more room to grow

Just be sure to test that machine = use it and if it seems hot remember you might only have 30 days to send it back
The 2 year extended warrantee is another good thing to get even though it might push the price up a little.

Janesdf said:

If I am correct, I need a ssd the same size as the hard drive?

The same capacity? No.

The same dimensions in inches? Probably.

This is the only win-7 machine I believe I posted,
Amazon.com : Dell Latitude E5410 Laptop WEBCAM - Core i3 2.26ghz - 3GB DDR3 - 160GB - DVDRW - Windows 7 Home 32bit (Certified Refurbished) : Computers & Accessories

Downgrading usually messes up everything which might explain what happen to your other laptop,
Plus will cost you for an additional win-7 install too unless I'm missing some thing there ?