What I look for in a good mixer are:
A comfortable grip (I'm sure we've all have painful wrists after whisking up eggs whites or similar)
Well made, sturdy parts. (some whisks are strong others are just thin wire.)
Variable speed control
The Watt rating.
To answer your No.2 question, watts are important. They measure how much power the machine uses. Higher watts will mean the machine does more of the work and your wrist does less of the work. The cheapest machines on the market are only good for whipping light foods like cream and mayonnaise. If you plan on mixing up heavy fruit cakes or bread doughs, you should choose a higher wattage machine.
Paying for a good quality machine, if you can afford it, will save you money in the end. My mother has had the same hand mixer for over 35 years. She paid for a good one and it lasted. I however, like you, opted for a low price machine because I was on a budget, I am now on my third and this time I bought a sturdier model and I am much more satisfied with it. If I'd bought the better machine at first, I'd have saved myself £50 or so over the time.
I've got one of the professional series Kitchen Aids (and my parents have the traditional version to compare and contrast). If money isn't a huge deal, then getting the professional series comes with a more powerful motor and a heavier base that will stop the mixer from "walking" around the counter if you put dough or something dense in the mixer.
One odd thing that I didn't expect is that the mixer is really inconvenient for small things. It can whip up like a triple batch of whipped potatoes, but it has the darnest time whipping up a little bit of whip cream or a little merangue.
Kitchen Aide's attachments are pretty rock solid... One of our more "fun" attachments that we have is the ice cream bowl. Freeze the bowl, dump the ingredients and set the paddle on low. Again, with the professional model, the motor is strong enough to churn a bit longer than traditional other ice cream churns...
IMO, if you're a big dough cook, go w/ the stronger model, otherwise the smaller models should be great.
EDIT
In an interesting note, my sister has one of the smaller models and it did infact "walk" off the counter a few weeks ago... The whole machine went crashing to the ground... She picked it up, dusted it off, but the only damage was a small piece of plastic broke off... Gotta give it to Kitchen Aide
Best Answer
Unfortunately not. The Delonghi/Kenwood mixers have a different attachment interface than KitchenAid, with a hexagonal shank and a different mechanism for retaining the attachment.