The biggest problem now is price. Two doors are going through the roof whilst more doors (four doors) although not quite as desirable are also increasing in price.
From an engine point of view the 1300 Crossflow although a good engine needs serious money throwing at it to get a decent HP outcome.
1600 pre 711 M s engine are ok but the 1600 711 M Crossflows are the stronger engine. Easily bored out to 1700 and with a set of twin 40 DCOE's 120 BHP is very easy to get with light engine work. After that a decent head, balanced bottom end, bigger carbs etc will produce more. These engines with money spent on them can produce some serious horsepower and a capacity increase over and above 1700cc will help.
Pintos are a straight forward fit but you will need a the ali sump and bellhousing off a RS2000 and of course the gearbox. get a type 9 5 speed. Sky's the limit with these engines and with money spent (a lot) you can get in excess of 220 bhp normally aspirated.
Now for the other options, BDA yes please but you will need a mortgage to buy and run one of these, Twinks (Lotus Twin Cam) again yes please but still serious money.
Other popular engine fits are Zetecs and Duratecs with big numbers available from both. Vauxhall XE is another with big power available as well. SBD is the company to talk to about them.
Then the Daddy, Cosworth powered and not the BDA. As much horsepower as your wallet can provide.
Then there is the gearboxes, big numbers will need decent boxes and certainly modified so money to be spent there. Rear axle. There is a wide range of ratios for the English axle and can handle reasonable power. But when hunting big numbers then a Baby Atlas is the way forward although not cheap.
Another option is the Volvo 240 rear axle as well.
So are they reliable, yes they can be, thing like electric dizzys etc help no end. Comfortable? well thats down to what you fit under the bonnet, suspension and so on. I used to run a modified MKII RS2000 as my daily (2.1 Twin 45's five speed box etc) and travelled all over the uk and Germany with it. Only once in 12 years did it get towed to a garage and that was due to wheel bearing failure.